Create Apache POI Custom Color

Excel template path

String excelTemplate = "C:/Export-Generic.xls";

Create HSSFWorkbook

HSSFWorkbook workbook = new HSSFWorkbook(new FileInputStream(excelTemplate));

Create HSSFPalette (Represents a workbook color palette.)

HSSFPalette palette = workbook.getCustomPalette();

Sets the color at the given offset

Parameters:

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

Note: As I know of default colors occupy 0-40. Therefor, custom colors needs start at 41. Custom color that I've chosen is #CADDFF it's RGB values are R = 202, G = 221, and B = 255. http://html-color-codes.info helped me to get the RGB colors.

palette.setColorAtIndex(new Byte((byte) 41), new Byte((byte) 202), new Byte((byte) 221), new Byte((byte) 255));

Now we have custom color palette. We can use this to set our style.
Create style

HSSFCellStyle style = workbook.createCellStyle();

Apply the our custom color to cell background

style.setFillForegroundColor(palette.getColor(41).getIndex());

Happy Coding :-)