目睹物而知形,然形非色而不见;色非光不见.故色必资乎光;昼资乎日,夜资乎月星与火.光盛则色显,光微则色隐.色依光而显其形:色浓则明,色淡则藏;色立乎异,则相得益彰,色傍乎同,则若存若亡.

"目睹物而知形"是见到物而知其形状.
We know the shape of a thing when we see it,but its
shape will not been seen if it has no color. The color
of a thing will not be seen if there is no light.
资乎: depend on; help
So the color for us to see has to depend on light, just
as the day on the sun; the night on the moon and stars,
and fire. When the light is strong,the color becomes
apparent; and when the light is dim,the color is obscure.
依:(色依光...)by means of; by.
A thing shows its shape by its color: it is conspicuous
when its color is deep; it is invisible when its color is
light.
立乎:存在.be.
There will be a striking contrast if the colors of the
things are quite different while their existence will
not be felt if their colors are almost the same.