When I bought my little rose bush the roses were red, now they're getting paler and turning white, why?

It sounds as if you had grafted rose bushes. A rose bush bearing the kind of roses you want is grafted onto the roots of a wild rose. This works good if all goes well. The much hardier roots supply the grafted rose bush and helps keep it healthier. But if something goes wrong the top rose bush could die. Since the roots actually belong to a different rose bush, which survived, then the rose bush that grows will be the one that originally went with the roots. That's one explanation and it happens a lot.