How do I use rgb colors for buttons?

In one of my upcoming games I have an HTML button which I want to fade from one color to the next. I used events and variables to do this using rgb. It was basically:

Set CSS Style "background-color" to rgb(0,0,ColorRandomizer).

However, when I tried to do this, it said it didn't accept numbers. I think it wants color names, but I can't really do that with my randomizing code. How do I use rgb numbers in the background-color CSS style for HTML buttons?

Thanks in advance!

The value needs to be a string.