I grow a lot of dahlias, mostly collarettes - I think I have around thirty cultivars. Their open flowers are very attractive to bees of all sorts. At present the honey bees are collecting both nectar and pollen.
Bumblebees love the dahlias as well. At this time of year most of the visitors are males or new queens so they are interested in nectar rather than pollen.
The bees visit all the flowers, regardless of colour. Although I have read that bees don't see red flowers easily, they seem to have no problem finding these. It may be, of course, that the flowers look different in ultraviolet light and even the red flowers have contrasting inner petals, perhaps as a guide to the bees.