I have noticed that colors on multisampled renderbuffers are not accurate. It's probably because of pixels "blending" with each other, though I have no idea on how renderbuffers work so this is more of a guess. Anyway, here's a picture of 3 cubes, each rendered with a different sampling method. Top: no multisampling, left: 2 samples, right: 4 samples.
Is there a way of making them look the same?
Edit: I'm on a GL3+ context so I hope I can counter this effect on the pixel shader. No more ideas so far.