If it is pure talc, there shouldn't be a problem with yellowing. However, most baby powder products contain corn starch, which translates to food for micro-organisms, house lice, various small beetles, etc. Talc is also not the best thing in the world to inhale, using a dust mask might not be a bad idea.
There many other products available for making snow. Plaster of Paris is commonly used, as is powdered Alum (Aluminium Sulfate) and Epsom Salt (Magnesium Sulfate). Companies like Woodland Scenics, Hudson & Allen and others also make commercial snow products. Just stay away from anything organic, as this will sooner or later decompose or serve as food for some unwanted pests. I would also tend to stay away from Baking Powder, as it too contains corn starch, as well as Baking Soda, which tends to decompose over time.
Osprey's book; Terrain Modelling, ISBN: 1-84176-062-5, has a whole chapter on modeling ice and snow. An excellent book on diorama groundwork techniques.