layers superposition

Hi!

I started to use vectorial layers for a project where I need superposition of colors. I made a background on a layer (light grey), then add another vectorial layer above. I understand that when a layer (with vectorial brush) is above, it is supposed to put the color above also. Can you tell me why, on this picture, the third layer influence the darker black (makes it darker). The goal here is to make a simple superposition of colors. I don't know if my question is clear. How vould I do that (i'm an absolute beginner).