From an XSSFCellStyle you can get the XSSFFont. From that, you can get the XSSFColor. Finally, from the XSSFColor, you can get the colour information as ARGB Hex, RGB with Tint etc.
Your code would look something like:
XSSFCellStyle style = cell.getCellStyle();
XSSFFont font = style.getFont();
XSSFColor colour = font.getXSSFColour();
System.out.println("The colour is " + colour.getARGBHex());