Hey! I did figure *something* out, so for me it affects just the Player -
In the Player Create Event, I put:
//shader
pos_uni = shader_get_uniform(sh_hue, "Position"); // control shader
// -1 - min, 1 - max
pos = 0;
and then (still in the Player object) I have an enemy Collision Event, so when Player takes damage I have:
// Take damage
pos = 2; //references Hue Shader
^ this alters the colour of the Player, but the whole colour (ie. white bits also change hue). I haven't had a play around to see if I can get it to only change certain parts of the sprite, but maybe this'll help!