Color of font is not set properly by call to method XSSFFont.setColor(XSSFColor). [styles.xml] It sets 'rgb' attribute of 'color' element without an alpha channel but 'rgb' attribute is of type ST_UnsignedIntHex which has length 4 (ARGB in hexadecimal format). [XSSFFont.java] Guess the problem is in XSSFFont's method setColor(XSSFColor) at the following line: ctColor.setRgb(color.getRGB()); where a call to getRGB() should be replaced by getARGB().
This sounds like a duplicate of #54373 or at least it sounds closely related.
We improved support for transparency/alpha-channel via r1907106 and r1907105 which hopefully also solves this issue.