The problem with this approach is, that you can disable a color channel so that it's not going to be written. But what is already in there remains. When writing a (100,120,180)-pixel with only red enabled to a (190,60,80)-pixel in the framebuffer the resulting pixel is (100,60,80), i.e. the new value for red, the old ones for all other components. That's what causes the "blending", because you are actually mixing the colors.
However, i'm not really sure how you want your rendering to look like. Maybe you can fake a screen shot that shows the final result so that one can imagine what's your goal.