The colours of Stripes and Pocket can be a pain sometimes for mismatch in any pony those green/blue shades but I would not say its common for them and if it is its not major. For returning, its going to depend on how the seller listed the ponies as to whether or not a return is reasonable. But if it annoys you, if its quite significantly more than could be expected in a used toy and if it was not disclosed by the seller, then I certainly think you should speak to the seller about it and ask for a return. Its a personal judgement but if you spend money and are not happy then you need to do something about it.
They are not that uncommon on ebay, you will certainly find them again easy enough if you keep an eagle eye out.