I came across this Mason Bee nest box, which has appealing--at least to my eyes-- two-tone trays. But I wonder if bees feel the same. Assuming all trays were installed at the same time, and that bees have no upper/lower preference, the nesters seem to prefer all-brown or half-brown nesting holes, or at the very least avoid like crazy the zazzy colour combinations. Very conservative little creatures.
Further research is required.