Which color cannot be produced in Roses, even through genetic alteration?

Blue cannot be produced in roses through genetic alteration because roses do not contain any naturally occurring blue pigments. The genes responsible for producing blue colors in other flowers are not present in roses. While genetic engineering can introduce genes from other species to alter the color of roses, no naturally occurring blue pigment gene has been found that can be successfully transferred to roses. Therefore, producing blue roses remains a challenge even through genetic alteration.