Which of the following colours cannot be seen by bees?

a) Red

b) Yellow

c) Green

d) Ultra-violet

Bees do not see red and have a hard time distinguishing it from surrounding green leaf backgrounds.  Bees that frequent red flowers are either perceive them in color they can see, or the red flower is not being lost against a green background.  Even though bees don't see red, they can see other reddish wavelengths such as orange and yellow.