It is fine to appear opaque with this ability! He will also be able to mimic texture though he will not gain any extra solidity.

As for altering the color, this requires a slightly more complex answer. With a simple backdrop (such as a plain white wall) this would be fairly easy. With something more complex it will take more time to imitate the pattern, so if he were to try and imitate a floor of multicolored gravel then he would need to stay still in order to maintain the effect. The more complex imitations would also tire him and increase his hunger very quickly, similar to a shade's invisibility.