I know that anthocyanins are a class of compounds responsible for the purple colors found in flower petals.
Anthocyanins (also anthocyans; from Greek: ἀνθός (anthos) = flower + κυανός (kyanos) = blue) are water-soluble vacuolar pigments that may appear red, purple, or blue depending on the pH. (same reference as above)
The anthocyanins, when extracted from flower petals with water, can be used as acid/base indicators, producing red colors in acid and blue in neutral solutions (in basic solutions they turn green, yellow, and then become colorless). When I've done the extractions, some red petals produce a red color in acid and maintain the red color at higher pH than purple flowers (see plate below), but some red flowers produce colors that seem to follow the normal progression.
Row 2 from the top was a purple flower, row 3 a red flower and row 4 also a red flower. (pH values start at 2 on the left and increase by 1 pH unit as you move right by adding the water extracts to buffer samples in the wells.)
So, are there other classes of compounds responsible for the red and yellow colors found in flowers? Can the red colors that give a "normal" anthocyanin pH response arise from pigments held in an acidic environment in the plant? (That may be a biology question, but I'm interested in the chemistry side of where the color comes from.)