http://poi.apache.org/: Apache POI - Java API To Access Microsoft Format Files (Apache Software Foundation) 
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package org.apache.poi.hssf.usermodel;

import java.util.Locale;

import org.apache.poi.hssf.record.PaletteRecord;
import org.apache.poi.hssf.util.HSSFColor;
import org.apache.poi.hssf.util.HSSFColor.HSSFColorPredefined;


Represents a workbook color palette.
Internally, the XLS format refers to colors using an offset into the palette
record.  Thus, the first color in the palette has the index 0x8, the second
has the index 0x9, etc. through 0x40

/**
 * Represents a workbook color palette.
 * Internally, the XLS format refers to colors using an offset into the palette
 * record.  Thus, the first color in the palette has the index 0x8, the second
 * has the index 0x9, etc. through 0x40
 */
public final class HSSFPalette {
    private PaletteRecord _palette;

    protected HSSFPalette(PaletteRecord palette)
    {
        _palette = palette;
    }

    
Retrieves the color at a given index

Params:
  • index – the palette index, between 0x8 to 0x40 inclusive
Returns:the color, or null if the index is not populated
/**
     * Retrieves the color at a given index
     *
     * @param index the palette index, between 0x8 to 0x40 inclusive
     * @return the color, or null if the index is not populated
     */
    public HSSFColor getColor(short index)
    {
        //Handle the special AUTOMATIC case
        if (index == HSSFColorPredefined.AUTOMATIC.getIndex()) {
            return HSSFColorPredefined.AUTOMATIC.getColor();
        }
        byte[] b = _palette.getColor(index);
        return (b == null) ? null : new CustomColor(index, b);
    }

    
Retrieves the color at a given index

Params:
  • index – the palette index, between 0x8 to 0x40 inclusive
Returns:the color, or null if the index is not populated
/**
     * Retrieves the color at a given index
     *
     * @param index the palette index, between 0x8 to 0x40 inclusive
     * @return the color, or null if the index is not populated
     */
    public HSSFColor getColor(int index) {
    	return getColor((short)index);
    }

    
Finds the first occurrence of a given color

Params:
  • red – the RGB red component, between 0 and 255 inclusive
  • green – the RGB green component, between 0 and 255 inclusive
  • blue – the RGB blue component, between 0 and 255 inclusive
Returns:the color, or null if the color does not exist in this palette
/**
     * Finds the first occurrence of a given color
     *
     * @param red the RGB red component, between 0 and 255 inclusive
     * @param green the RGB green component, between 0 and 255 inclusive
     * @param blue the RGB blue component, between 0 and 255 inclusive
     * @return the color, or null if the color does not exist in this palette
     */
    public HSSFColor findColor(byte red, byte green, byte blue)
    {
        byte[] b = _palette.getColor(PaletteRecord.FIRST_COLOR_INDEX);
        for (short i = PaletteRecord.FIRST_COLOR_INDEX; b != null;
            b = _palette.getColor(++i))
        {
            if (b[0] == red && b[1] == green && b[2] == blue)
            {
                return new CustomColor(i, b);
            }
        }
        return null;
    }

    
Finds the closest matching color in the custom palette.  The
method for finding the distance between the colors is fairly
primative.

Params:
  • red –   The red component of the color to match.
  • green – The green component of the color to match.
  • blue –  The blue component of the color to match.
Returns: The closest color or null if there are no custom
         colors currently defined.
/**
     * Finds the closest matching color in the custom palette.  The
     * method for finding the distance between the colors is fairly
     * primative.
     *
     * @param red   The red component of the color to match.
     * @param green The green component of the color to match.
     * @param blue  The blue component of the color to match.
     * @return  The closest color or null if there are no custom
     *          colors currently defined.
     */
    public HSSFColor findSimilarColor(byte red, byte green, byte blue) {
    	return findSimilarColor(unsignedInt(red), unsignedInt(green), unsignedInt(blue));
    }
    
Finds the closest matching color in the custom palette.  The
method for finding the distance between the colors is fairly
primative.

Params:
  • red –   The red component of the color to match.
  • green – The green component of the color to match.
  • blue –  The blue component of the color to match.
Returns: The closest color or null if there are no custom
         colors currently defined.
/**
     * Finds the closest matching color in the custom palette.  The
     * method for finding the distance between the colors is fairly
     * primative.
     *
     * @param red   The red component of the color to match.
     * @param green The green component of the color to match.
     * @param blue  The blue component of the color to match.
     * @return  The closest color or null if there are no custom
     *          colors currently defined.
     */
    public HSSFColor findSimilarColor(int red, int green, int blue) {
        HSSFColor result = null;
        int minColorDistance = Integer.MAX_VALUE;
        byte[] b = _palette.getColor(PaletteRecord.FIRST_COLOR_INDEX);
        for (short i = PaletteRecord.FIRST_COLOR_INDEX; b != null;
            b = _palette.getColor(++i))
        {
            int colorDistance = Math.abs(red - unsignedInt(b[0])) +
            	Math.abs(green - unsignedInt(b[1])) +
            	Math.abs(blue - unsignedInt(b[2]));
            if (colorDistance < minColorDistance)
            {
                minColorDistance = colorDistance;
                result = getColor(i);
            }
        }
        return result;
    }
    
    
Turn a byte of between -127 and 127 into something between
 0 and 255, so distance calculations work as expected.

/**
     * Turn a byte of between -127 and 127 into something between
     *  0 and 255, so distance calculations work as expected.
     */
    private int unsignedInt(byte b) {
    	return 0xFF & b;
    }

    
Sets the color at the given offset

Params:
  • index – the palette index, between 0x8 to 0x40 inclusive
  • red – the RGB red component, between 0 and 255 inclusive
  • green – the RGB green component, between 0 and 255 inclusive
  • blue – the RGB blue component, between 0 and 255 inclusive
/**
     * Sets the color at the given offset
     *
     * @param index the palette index, between 0x8 to 0x40 inclusive
     * @param red the RGB red component, between 0 and 255 inclusive
     * @param green the RGB green component, between 0 and 255 inclusive
     * @param blue the RGB blue component, between 0 and 255 inclusive
     */
    public void setColorAtIndex(short index, byte red, byte green, byte blue)
    {
        _palette.setColor(index, red, green, blue);
    }

    
Adds a new color into an empty color slot.

Params:
  • red –       The red component
  • green –     The green component
  • blue –      The blue component
Throws:
Returns: The new custom color.
/**
     * Adds a new color into an empty color slot.
     * @param red       The red component
     * @param green     The green component
     * @param blue      The blue component
     *
     * @return  The new custom color.
     *
     * @throws RuntimeException if there are more more free color indexes.
     */
    public HSSFColor addColor( byte red, byte green, byte blue )
    {
        byte[] b = _palette.getColor(PaletteRecord.FIRST_COLOR_INDEX);
        short i;
        for (i = PaletteRecord.FIRST_COLOR_INDEX; i < PaletteRecord.STANDARD_PALETTE_SIZE + PaletteRecord.FIRST_COLOR_INDEX; b = _palette.getColor(++i))
        {
            if (b == null)
            {
                setColorAtIndex( i, red, green, blue );
                return getColor(i);
            }
        }
        throw new RuntimeException("Could not find free color index");
    }

    private static final class CustomColor extends HSSFColor {
        private short _byteOffset;
        private byte _red;
        private byte _green;
        private byte _blue;

        public CustomColor(short byteOffset, byte[] colors)
        {
            this(byteOffset, colors[0], colors[1], colors[2]);
        }

        private CustomColor(short byteOffset, byte red, byte green, byte blue)
        {
            _byteOffset = byteOffset;
            _red = red;
            _green = green;
            _blue = blue;
        }

        @Override
        public short getIndex()
        {
            return _byteOffset;
        }

        @Override
        public short[] getTriplet()
        {
            return new short[]
            {
                (short) (_red   & 0xff),
                (short) (_green & 0xff),
                (short) (_blue  & 0xff)
            };
        }

        @Override
        public String getHexString()
        {
            StringBuffer sb = new StringBuffer();
            sb.append(getGnumericPart(_red));
            sb.append(':');
            sb.append(getGnumericPart(_green));
            sb.append(':');
            sb.append(getGnumericPart(_blue));
            return sb.toString();
        }

        private String getGnumericPart(byte color)
        {
            String s;
            if (color == 0)
            {
                s = "0";
            }
            else
            {
                int c = color & 0xff; //as unsigned
                c = (c << 8) | c; //pad to 16-bit
                s = Integer.toHexString(c).toUpperCase(Locale.ROOT);
                while (s.length() < 4)
                {
                    s = "0" + s;
                }
            }
            return s;
        }
    }
}