Is it the nectar, pollen or colour that attracts bees to flowers. May be it is all three!
Just watch bees swarm over Thyme when in flower.

Cistus only seem to flower for one day then the petals drop. Resting in the sun this Bee seems content with life.

Foxgloves have long tubular flowers so the Bee has to crawl right in to get the nectar. This moves pollen from flower to flower and Foxgloves then produce thousands of small powdery seeds.

Not a strictly purple colour but this image has appealed to me as the flower appealed to the Bee.

So the Bee was too quick for me this early in the year. In truth this Rhododendron did not attract swarms of bees back in April

Other Resources

Royal Horticultural Society RHS ‘Gardening for All’
National Council for Conservation of Plants and Gardens ‘Conservation through Cultivation.’
Garden Organic National Charity for Organic Gardening.
BBC Gardening