Why is the western toad darker on the top and lighter on the bottom?

Bufo boreas, or the western toad has
an interesting color pattern. The toads have a dorsal stripe running along their back
which is white or cream. Their upper body has green or gray coloration with dark
blotches and warty skin. The underside or ventral surface is light in coloration. The
difference in the upper and lower surface are adaptations for survival. Toads can
venture on land and therefore, must be able to blend in to avoid predation. However,
when it is time to mate, as in all Amphibians, they must return to the water. If toads
are near the surface of a pond, any organisms below them looking up will not easily see
them due to their light colored underbelly, when the sun is above the pond as well.
Therefore, they are camoflauged and thus protected above and below from
predators.