Yes, you can change the color of anything in Rainmeter. What skin are you trying to change?
The code sometimes looks like this...
FontColor=0,0,0,100 (black, semi-transparent)
The first 3 numbers are RGB (red,green,blue). The 4th number is for transparency. 0 for transparent - 255 for nontransparent. You can use any number in between.