When i was much younger, in a class somewhere, i saw pictures of flowers taken with somekind of UV filter. Different species of flowers had different patterns when viewed with UV. So a human sees solid yellow flowers, but bees see patterned yellow flowers. The thought at the time was that, flowering plants used this as a strategy to keep a bee interested in its species, creating a greater chance that its individual pollen will rub off on a flower of the same species. Supposedly in a garden full of different cucurbits, one can see an individual bee visiting all the cucumber blossoms then all the squash etc.