Water vapour absorbs light above our visibility range (UV) and from about 480 nm wavlength into the infrared part of the spectrum. That could comply with my suggested explanation. As usual, i have nothing written in english at hand, you might ask a physical chemist in your vicinity about that. As a cloud's reflection rate is quite high, it likely can have a significant absorption rate. The absorption is not complete, as a cloud does not consist of a layer of pure water vapor (in which case it would look blue), but of about 1 to 5 vol.% water vapor (about 100% relative humidity), plus the water drops. I'm not an expert for clouds, the athmosphere or the like, but from my spectroscopy courses (I dropped out halfway from a study of chemistry) I remember that water absorbs quite effectively even if present only in a small amount.