Phytochrome activity is thought to be quite well understood. This is the basic red (660nm) far-red (730nm) receptor which controls leaf size, stem elongation, seed germination in some species, and some photoperiodic effects such as flowering. The effects of relative levels of red and far-red light and the effects of the two types of light at different times are well studied.

Cryptochrome is one of several blue-sensitive receptors. It is not as well-studied, but is involved in phototropism (following the light) and aspects of the circadian rhythm.

It would probably be best to consider all blue-light responses rather than isolate only cryptochrome. Other important effects of blue light are the synthesis of chlorophyll and other pigments, the internal arrangement of leaf cells and chloroplasts, stomatal opening, and many subtle effects caused by hormones and even changes in gene expression. To put it very simply, plants won't grow well without some blue light.

Excluding photosynthesis from a discussion of plant lights is probably a bit limiting. The vast majority of the light needed by plants is for photosynthesis. All the other effects that have been studied are mediated well by relatively low levels of the appropriate light, so even if a particular function absolutely requires some wavelength of light it might need so little of it that almost any light source would be sufficient.