okay thanks - i found my mistake.
I chanced in my image-manipulation-program the opacity of the layers to "addition". Otherwise the layer above overwrites the r-g-b-values of the one beneath *facepalm*
I made a image, if someone have the same problem:

Now it works, thanks HeelX!


left side without additon. The red color has blue=0 and on the blue layer, blue is reset to 0 -> dark.
With addition the effect is avoided, as you can see on the right side, where I hided the red-channel.