Purple wheat grains are a promising source of anthocyanins and have been proposed as raw material for baking. However, little information is available about the effect of physiological and management variables on grain anthocyanins. This research aimed to (i) characterize the anthocyanin dynamic of grains set at different positions within the spikelet, (ii) evaluate the effect of different source–sink ratios on anthocyanins, and (iii) assess the response of anthocyanins to magnesium fertilization and harvest timing. Four source–sink ratios (pre- and post-anthesis spike halving, spike-shading and control) and two agronomic practices (Mg fertilization and early harvesting) were evaluated in field trials. Grain position affected grain weight and anthocyanin content, decreasing at distal positions. Both variables showed positive response to higher source–sink ratio. Pre-anthesis halving increased anthocyanin content by 54 and 37% (Exps. 1 and 2), while post-anthesis halving increased this trait by 31 and 29%. Source reduction decreased anthocyanin content by 26% and 51%. Changes in anthocyanin content associated with source–sink treatments and with grain positions were the result of different maximum anthocyanin content reached by physiological maturity. Magnesium fertilization and early harvesting increased anthocyanin content and concentration by 65 and 39%, respectively, seeming promising to increase anthocyanins in purple wheat.