4 years ago in r1295079, setTabColor(int colorIndex) was added to XSSFSheet, but no getter was added, nor a way to add a non-indexed color. Since Excel 2007+ supports sheet tabs to be any color, including themed or tinted colors, setTabColor(XSSFColor color) seems like a more appropriate method, along with a corresponding getter.
Added in r1739547. Added to SXSSFSheet in r1739548.
Update changelog in r1739589.