Setting colors to the value of a variable

I'd like to set the colors of my text and background according to the values of variables. Unfortunately, I can't figure out how to evaluate a variable inside custom style markup. The following code should illustrate what I am trying to do:

<<set $forecolor to "green">>
<<set $backcolor to "orange">>

@@color:$forecolor;background-color:$backcolor;This should be some $forecolor text on $backcolor background.@@

@@color:green;background-color:orange;This works but the last one didn't.@@

How can I achieve this?