Erm what my opinion is:
1st: their dick won't be as obvious as the lighter colored pants,
cyclist have to wear tight fitting pants so it won't much much or a
obstacles for the to cycle. Especially big dicks, it is even more
obvious 
2nd: something there is gease on the bicycle, it is normal because
you need to smoothen your bike. this gease won't be nice if come
contact with light colored pants.
hope it will solve your questions 
