Alfalfa Flowers
"Typical" alfalfa flowers are purple, but may have varying shades of white and yellow.  Normally flowers with shades of yellow indicate that some amount of Medicago falcata germplasm is present.   

Flowers are borne on racemes with 10 to 50 individual flowers. 

Butterflies as seen in this photo do little, if any, pollination of alfalfa flowers.  They feed on nectar without tripping alfalfa flowers.