A white rose cannot be produced in the G1 (first generation) of roses because the red rose will have a RR (R standing for red) allele combination. The pink rose will have a RW (R for red, W for white) allele combination. When crossed all combinations will have at least one R allele, meaning that no roses (in the first generation) will have a WW allele combination. WW is the only allele combination that produces white roses.
(look up punnet squares for more help)