By the genotypes provided, red would prove to be homozygous dominant. This would give the alleles RR which if crossed with another red flower, also RR, 100% of the offspring would be RR. Yellow would have to be heterozygous dominant, by convention. The flower's genotype would be Rr, which if crossed, would give you 25% red flowers, 50% yellow, and assumingly, 25% homozygous recessive which could produce a flower with a different phenotype (white, green. . .) This is known as Incomplete Dominance, which is distinguished by the heterozygous phenotype's expression being different than the homozygous dominant flower.