It is possible to modify the color of the flowers by mordern molecular breeding technologies.It is the anthocyanin pigments and its pattern that gives colors to the flowers.
The blue hues in flower is because of the presence of the pigment Delphinidin (Purple anthocyanin).It is a anthocyanidin.The precursor of the purple anthocyanin is dihydroflavonols (anthocyanin dihydrokaempferol -DHK.) Flavonoid 3',5'-hydroxylase (F3'5'H) is a cytochrome P450 enzyme (Cyt P450) that catalyzes the 3',5'-hydroxylation of dihydroflavonols.Roses lack this enzyme.
Earlier attempts to replace the gene for Delphinidin in roses had failed .Delphinidin gene cloned from petunia and expressed in roses created a dark burgundy rose ,not blue. So instead of modifying the color,scientists were looking for a genes/proteins involved in pathways before any color formation in roses. They found an enzyme dihydroflavinol reductase (DFR) which modifies the pigmentation from white to another color. In other words,until this enzyme acts, all the pigment molecules are colorless.
The solution to create a blue rose is obtaining a DFR mutant white rose. How to knock out the gene for the enzyme? The answer is in RNAi technology.
Using RNAi,they suppressed the DFR gene and cloned a new verson of Delphinidin in roses.
More on the technology here.