Flavonoid pigments are key determinants of flower colors. As absorption spectra offlavonoids are known to be severely pH-dependent, cellular pH will play a crucial rolein flower coloration. The flavonoids are concentrated in the vacuoles of the flowers’epidermal cells, and thus the pigments’ absorption spectra are modulated by thevacuolar pH. Here we study the pH dependence of flavonoid absorption spectra inextracts from flowers of two poppy speciesPapaver dubium(red) andMeconopsiscambrica(orange), and a white and redMandevilla sanderivariety. In the red poppyandMandevillaflowers, absorption spectra of the cyanidin- and pelargonidin-basedanthocyanins peak in the blue-green-wavelength range at low pH, but exhibit a distinctbathochromic shift at higher pH. This shift to longer wavelengths is not found for theblue-absorbing nudicaulin derivatives ofM. cambrica, which have a similar absorptionspectrum at low and high pH. The pH-dependent absorption changes of the whiteM. sanderi’s flavonoid remained restricted to the UV. An analysis of the spectra withlogistic functions suggests that the pH-dependent characteristics of the basic states offlavonols and anthocyanins are related. The implications of tuning of pH and pigmentabsorption spectra for studies on flower color evolution are discussed.