You may want to look at the Colormix1 and Colormix2 functions.
Change "By Measure" to "By Expression" in the dropdown menu.
In the expression window, type:
The first parameter in Colormix1 and Colormix2 must be a value between 0 and 1, so you'll need to reduce your expression to a range between those values.
Here is a good write-up on setting a custom color range and setting a theme for your apps.