I've hit a few snags with the 'Subtract' (PS 0 version) effect.
First, the effect has a different output in the level editor and in the actual .exe. Inside the level editor the effect renders appropriately - sprites with the 'Subtract' effect darken other sprites. In the .exe, the effect is rendered as if it was set to 'Additive' - sprites lighten other sprites.
Screenshot of this in action: Here
Secondly, the 'Subtract' effect changes how sprite opacity is handled inside/outside of the level editor. Inside the level editor, sprites with the 'Subtract' effect are rendered semi-transparent, even if their opacity = 100%. Outside of the level editor sprites with opacity = 100% are correctly rendered fully opaque.
Screenshot of this in action: Screenshot of this in action: Here
I've also attached a simple .cap file to demonstrate the effect.
If anyone sees anything obvious I'm doing wrong feel free to ridicule me. :]