Ultraviolet photograph of a Water Mint plant (Mentha aquatica) in flower. In UV the plant's flowers, normally light pink / lilac, appear much darker.
Bees and other insects can see part of the ultraviolet spectrum, and it is believed that some plants use ultraviolet markings to help guide insects to where the nectar / pollen is located. In turn, this helps the plant get pollinated.
Colour modified in PS CS5