mireazma

I understand but I am refreshing the surface manually and every step the surface is drawn from complete zero. So I imitate GM onscreen drawing. I suspect (unsure) that the problem is in GM blending mode.

1 - If I simply draw an opaque black background and then a semi-transparent white spot on top, GM blends them, right?
2 - If I draw the opaque black background and then the semi-transparent white spot on top into a surface, at drawing onscreen, this should look exactly like in (1), shouldn't it? As long as GM refreshes the screen and I am drawing anew. Am I missing anything?

Well, you put that code into a script called "random_color" and call it like this:

draw_sprite_ext(sprite_index, image_index, x, y, image_angle, random_color(), alpha);

About matching colors, you have to think numbers, not colors so have 3 vars red, green, blue; i.e if you input a RED value that matches "red" have GM do the thing. That's pretty obvious but still...; and if you want to do something based on how close you are to the color you can use

if(abs(argument0 - red) + abs(argument1 - green) + abs(argument2 - blue) < argument3)
 return 1;
else
 return 0;

in the script, after the code you got; you'd pass in the RED, GREEN, BLUE and threshold and return either the color to draw the sprite or, the "go" for doing something externally.

Does it happen when you rotate the instance?

Is the path regularly shaped? Not that I could help but it worth mentioning.

I can't describe the frustration I had once with a similar issue. It took me several days until I knocked it down. I don't say it's your case too but it might be. I don't know about Studio but the legacy GM seem not to have a consistent order of creating objects (or at least so I took it). Merely creating instances in an express, obvious order has solved it. Until then I was residing on instances id -- bad way.