One thing I like Stella debug feature, it easy to follow the code. You can hit the scan+1 until you see the picture being developed on the left top window. You can click on the bigger window right of it and use the arrow key to move around. Once you see the desire scanline you want to change, then you can use the step function to follow the code and when register COLOPF(Color Playfield) get loaded with a value.
When you see, STA.X, it is loading a register from an pointer. LDA COLOPF push the value to register color playfield. So the pointer LF1EC is the byte list of playfield color. You can change that by clicking on the byte, and enter new data and then push enter. For example,
I'm not sure if this list only for that level or is there more color tables. So this one works for this level.
Hopefully this helps you out.
EDIT: The game have an 2nd list of colors for one of the game variation.
Edited by Kiwi, Wed Jun 21, 2017 5:55 PM.