The allele would be called codominant
When a dominant and recessive allele exist in an organism, only the dominant allele will be expressed. Dominant allele always mask the recessive because the gene code a protein while recessive gene doesn't. When both gene code a protein, they will be codominant and both gene will be expressed. The result could be a mixed effect. In this case, the purple is mixed color of red and blue.
It happens during the s phase
