Not a marketing expert, but from some graphic design research and experience suggests that "solid colours" (whether the whole cover is one single colour), suggests a sense of either youth or simplicity, if bright and somewhat authoritative, if dark. On the other hand, gradient colours are generally considered more artistic and "look better", but on the whole, less professional and less appealing in a book, what do you guys say?
I'm doing a design right now and I'm seriously considering either solid colour or gradient fill.
Btw, FunkyDucky, it was my idea to colour code books based on subjects the way Maths Quest, Essential, Heinmann, NEAP and basically every other company do =.="