Out of curiousity, what puts a color in a rose?

The acidic form of anthocyanin.

The base form of anthocyanin makes cornflowers blue, and is also the case for hydrangea, strawberries and I think red cabaggages.

The acidic form of anthocyanin I assume leads to the reds while the base form leads to the blues.

Other chemicals are present as well, thus we get a wide variety of color excluding true black and (if I recall correctly) Blue. Ironic considering that cornflowers are blue and share the same molecule with the rose: anthocyanin. However the acid/base (PH) is what does the trick.

Anyway, more on the science: http://www.expasy.org/spotlight/back_issues/sptlt063.shtml