The first thing you need to do is create a new color resource in your SkinInfo.xml.
code:
<New>
<Colors>
<Color Id="59100">
<I>200</I> <!-- Intensity based on user selected color -->
<A>255</A> <!-- Alpha -->
</Color>
</Colors>
.....
</New>
Then whenever you want to use your new color you can just call on it with rcclr. For example.
code:
Element [id=atom(blah)] {
foreground: rcclr(59100); // Calls on color 59100
}
or
code:
<Element id=atom(blah) foreground=rcclr(59100)/>
Of course one thing to note is that the color will be gray unless the user has selected a color so please be aware of that.