Subject: Why do cave animals evolve to become albinos?

Many cave-dwelling animals are albinos decended from colored
above-ground ancestors.  It would seem that there is some 
selective pressure in caves that favor albinos over colored 
animals, otherwise cave animals would just have retained their 
ancestors' colors.  But caves are lightless environments, so I 
don't see how there could be such selective pressures that favor 
one color over another.

Why didn't cave animals retain the color of their ancestors?  Why
did they evolve from colored to colorless in an environment where 
color is a seemingly unimportant selective criteria?

Re: Why do cave animals evolve to become albinos?