I'm working with an application that often benefits from partially transparent colors. These colors are often based on existing colors but with a different transparency value. It would be very nice if these colors could be derived similar to what can be done for creating brighter and darker colors with derive(color, %).

Workaround:
Define colors like:
-c-blue: rgba(173, 216, 230, 1.0);
-c-blue-70: rgba(173, 216, 230, 0.7);
-c-blue-50: rgba(173, 216, 230, 0.5);

As you can see, having to respecify the entire color just to add some transparency is cumbersome.

Possible solutions:
1) Introduce a 3rd optional parameter to derive(): derive(color, brightness-adjustment, transparency-adjustment)
2) Introduce a new function... ehr, derivea (from rgba), deriveTransparency, tderive...