is it possible to replace the existing LEDs for custom colours?
Not really – it’s not discrete LED components underneath the segments, but dies and bonds straight onto the board.
This would be an even better product if the colours went from red to green! Any chance of this?
The company that makes them seems to allow them to be produced with one of four colors (red/orange, yellow, green and blue – ah, if only they offered white) for each quadrant. But whether they stock that configuration, and how much it would cost to get that (either piecewise or for SparkFun to get a bunch in one go) is not mentioned. If you’ve got a steady hand you could splice a red and a green together to your desired configuration ;)
Alternatively, you could check out the Rotary Encoder LED Ring Breakout Boards. Those do use discrete LED components that you could swap out.