How does one red flower bloom on a pink bush?

Most commercially sold roses are grafted. If someone chose to, they could graft several branches that would each bear a separate color on a bush. Very occasionally a stem of a plant will produce a bloom of a different color or shape to the rest of the plant. This is called a 'sport' and is one way in which new varieties occur. Cuttings can be propagated from the unusual stem to keep the variety going. Some plants are more prone to sport than others.