I removed the cover completely and I took pictures with the N900. Took the same pictures with the N97 and the N97 simply has MUCH better colours even with the cover off. It's not just about the cover.

The N900 uses a warmer colour palette in general. Viewing images on the N900 and the N97, you can see the same images look warmer on the N900. I think the N900's camera is calibrated so that taking images on it and viewing them on the phone would produce a "normal" quality. PIctures viewed elsewhere though would look dry.