It's pretty versatile. Only one of the functions can change opacity though--that's RGBA(). The next step to improving color ability is by having a Function for adjusting the opacity. I tried with ColorFade, but it only turns things black/white and is not able to affect transparency.
I think a Function may be better than a Property because it could fit in existing properties for Fill and Color.
I've leveraged this for the opacity of my disabled button fill so that the user can read the button text even when the button is disabled (I set it at 80% for example). Let me know if you need more syntax examples.