People of all classes would be using the same church, and church paintings were often very, very bright. The Episcopal Museum in Vic, Catalonia, holds some medieval paintings that have incredibly bright colors. We take pretty bright lighting for granted, but normal natural light is often pretty low. As a church space would often be dim, bright paintings could be seen better. MartinGreyWolf indicates a similar taste for brightness in dyes and clothes.

For pigments, it is true that some of the brighter ones could be expensive- like vermilion, and ultramarine- and some very reactive and impermanent- like verdigis green or yellow orpiment. It would have been very costly to paint room with ultramarine ( maybe cheaper to put gold leaf over it), and it was very tricky to know how to apply orpiment without it reacting with other colors ( like turning the verdigris green black). The duller earth colors- like Venetian red, yellow ochre, burnt umber- were easier to find and use, so cheaper and more common. But the expensive pigments would still find their way into things like wall paintings. Here's a nice portal site to English medieval wall paintings that gives a good idea: even here, a little vermilion or other more expensive color can be seen- though not a lot. In short, it looks like a far better case could be made that natural light tended to create a taste for brighter colors and cost limited their use more than some class distinction.