In XPP you can use the color property to define the foreground color of the text.
You can do something like this:
para {
     color: red;
}

or you can do something like:
para {
     color: -xppc(19);
}

The named colors are limited to the 9 main colors that XPP knows about.
As for the xppc() colors, you will first have to set up the color in the color spec before you can use it.

But CSS allows you to do so much more, you can define colors in the following ways:

• Hexadecimal colors
• Hexadecimal colors with transparency
• RGB colors
• RGBA colors
• HSL colors
• HSLA colors
• Predefined/Cross-browser color names
  
  

Adding this to the XPP CSS implementation would be a big step forward in making the XPP more user friendly and more in line with the modern world.
(not all PDFs that are created by XPP are used for printing purposes, more and more PDFs are used as an online medium)
It would be more user friendly as it does away with the nuisance of having to setup a color in the color spec and then trying to remember the number you assigned to it.
It would also allow to copy paste any color specification you can find into your CSS spec.

And last but not least, I would not mind if this new way of defining colors is only supported when creating PDFs and not in the PostScript world.
Which means it would only be available when you use Direct PDF to create the PDF output.
(which probably would make it easier for engineering to implement it?)