Abstract of some interesting results in bee preferences from:
Artz, D.R., Allan, M.J., Wardell, G.I., Pitts Singer, T. 2014. Influence of nest box color and release sites on Osmia lignaria (Hymenoptera: Megachilidae) reproductive success in a commercial almond orchard. Journal of Economic Entomology. 107:2045-2054.
"Blue orchard bees had better reproduction in light blue nest boxes followed by red nest boxes and then yellow nest boxes."
"In the nesting site density and nest box color experiment, the number of nests, number of cells, and number of male and female cells produced by O. lignaria females was greatly influenced by the number of available cavities within nest boxes and nest box color. "
My mason bees use the triangle between green siding and white trim boards. My observation is they hover in front looking for the dark hole. Holes drilled in posts they favored the ones in the smooth light colored pealed surface over darker split surface. Crown bees suggests they also use smell to locate used holes.