1. Anthocyanin pigments have become a model trait for evolutionary ecology since they often provide adaptive benefits for plants. Anthocyanins have been traditionally quantified biochemically, or more recently using spectral reflectance. However, both methods require destructive sampling and can be labour intensive and challenging with small samples. Recent advances in digital photography and image processing make it the method of choice for measuring colour in the wild. Here, we use digital images as a quick, non-invasive method to estimate relative anthocyanin concentration among plants exhibiting colour variation. 2. By using a consumer-level digital camera and a free image processing toolbox, we extracted RGB values from digital images to generate colour indices. We tested petals, stems, pedicels and calyces of six species, which contain different types of anthocyanin pigments and exhibit different pigmentation patterns. Colour indices were assessed by their correlation to biochemically determined anthocyanin concentration. For comparison, we also calculated colour indices from spectral reflectance and tested the correlation with anthocyanin concentration. 3. Indices perform differently depending on the nature of the colour variation. For both digital images and spectral reflectance, the most accurate estimates of anthocyanin concentration emerge from anthocyanin content-chroma ratio (ACCR), anthocyanin-chroma basic (ACCB) and strength of green (S green) indices. Some colour indices derived from digital images and spectral reflectance strongly correlate with biochemically determined anthocyanin concentration, but the estimates from digital images performed better than spectral reflectance in terms of 2 and normalized root-mean-square error. This was particularly noticeable in a species with striped petals, but in the case of striped calyces both methods showed a comparable relationship with anthocyanin concentration. 4. Using digital images brings new opportunities to accurately quantify the anthocyanin concentration in both floral and vegetative tissues. This method is efficient, completely non-invasive, applicable to both uniform and patterned colour, and works with samples of any size.