different species that belong to the same genus may be fertile but their is a 50% chance of that.
Offspring produced by a Roseicollis and any of the eyering type lovebirds (Fischeri, Personatus, Lillianae, Nigrigenis) will be infertile.
Any offspring produced between two different species of the Eye-ring group will be fertile.
Again, none of this is recommended nor advised. If breeding lovebirds, unless you are an expert in mutations and looking to bring one to another species, then always breed same species with each other.