It is "possible" with the current technology but difficult and here's why. Spells work differently then creature and object models, it's easy to reskin a bear because you can always add a new bear skin to the list of texture files the single bear model can use. This is not the case with spells,
most spells have their own model for each recoloring, this is because for
most spell models there is no separate .blp (texture) files, instead the texture is included directly in the .m2 (model) file.
So, like people have already said, Blizz would have to make new models for each spell, not only is that a lot of file space it would take some time to do.
Secondly, a lot of spells are made up of multiple models, usually the casting, the in-flight, and the impact, sometimes more. The casting is almost always reused from something else and this would not be the biggest hurdle but it's something to keep in mind none the less.
Overall Blizzard is better off changing how spells work in their engine before adding huge additions like this, in other words "the technology isn't there."