I bred a few blue roses by placing two blue roses diagonally.
I successfully bred them without using fertilizers,
maybe fertilizers can increase the chances of breeding hybrids,
but I don't think is necessary.
You can increase your chances of getting hybrids by enacting the Beautiful Town Ordinance as well.
you need to get a hybrid red rose by breeding either orange x purple, orange x white, or red x purple roses, then breed the hybrid red rose with either another hybrid red or a orange rose.
fertilizer is not required, but it does speed up the breeding process, as with all hybrids