Imagine the leftmost A control prompt above, but all white.
That’s the style I used in my most popular app, Tic Tac Toe Puzzle.
I wanted to incorporate color, to match the button colors of Xbox or PlayStation controllers. This introduced a readability issue. The colors don’t read as well on different colored backgrounds.
I iterated on my control prompt design (from left to right) by adding color, inverting transparency, and ultimately landed on pure white on the right.