Maybe not if it was a very expensive toy, but I have done this already. I don't think it was a limited time thing, or even if the toys came from the same company, but I got the bent graduate and then bought the amethyst when it came out. I hate pink, so spending $15 on a toy I know I will love was worth it just to have a better color. It helped too to know that because glass isn't porous, I could give the bent graduate to a friend.
If the toy wasn't too pricey, yes. Or if the old toy was starting to show wear (or in the case of vibes, was losing power), then yes. Otherwise, no.
The only toys I ever considered getting two of to have a different color were the Silk Medium (it used to come in a fantastic shade of blue, but I think that's been discontinued) and the Bent Graduate (there's a cobalt blue version that Eden doesn't carry - I may still get it someday, and I prefer it over the blue Amethyst).