You could make the code like:
set variable:
"rgb("+str(102)+","+str(153)+","+str(255)+")"
OR
set variable:
"rgb("+str(global('Variable1'))+","+str(global('Variable2'))+","+str(global('Variable3'))+")"
FYI: The "str(Value)" means that the number is converted into a string.
Hope this helps