Created attachment 24605 [details]
patch for HSSFPalette.java and TestHSSFPalette.java

HSSFPalette.findSimilarColor considers a color value of 0 to be closer than 255 it is to 127.  Thus, for example,

HSSFWorkbook workbook = new HSSFWorkbook();
HSSFPalette palette = workbook.getCustomPalette();
HSSFColor color = palette.findSimilarColor(0,102,255);
short[] rgb = color.getTriplet();
System.out.println(java.util.Arrays.toString(rgb));

prints "[255,102,0]".  The input color is not unlike turquoise, but the output color is a light red!

The reason findSimilarColor behaves this way is because it implements the Manhattan distance in byte space.  byte is signed, and thus 255 is really -1, which is adjacent to 0.

A patch is enclosed which first converts the bytes to positive integers so that 255 is maximally far from 0.  Then the above example returns [0,102,204] which is much closer to the original [0,102,255].  The patch also includes additional tests for TestHSSFPalette.