You get this error because pallete is full. What you need to do is override preset color. Here is an example of function I'm using:
public HSSFColor setColor(HSSFWorkbook workbook, byte r,byte g, byte b){
HSSFPalette palette = workbook.getCustomPalette();
HSSFColor hssfColor = null;
try {
hssfColor= palette.findColor(r, g, b);
if (hssfColor == null ){
palette.setColorAtIndex(HSSFColor.LAVENDER.index, r, g,b);
hssfColor = palette.getColor(HSSFColor.LAVENDER.index);
}
} catch (Exception e) {
logger.error(e);
}
return hssfColor;
}
And later use it for background color:
HSSFColor lightGray = setColor(workbook,(byte) 0xE0, (byte)0xE0,(byte) 0xE0);
style2.setFillForegroundColor(lightGray.getIndex());
style2.setFillPattern(CellStyle.SOLID_FOREGROUND);