Seed coat colour is an important determinant of the visual quality of dry beans, as seeds are sold as a dry commodity. Phenolic compounds have a major effect on the colour of bean seeds. The objectives of the study were to determine the changes in phenolic compounds during seed development and in whole seeds of yellow bean genotypes with contrasting seed coat colour, and the effects of storage temperature and duration on seed phenolics and colour. Condensed tannin, phenolic acid, flavonoids, and antioxidant activity were observed as early as 10 d after flowering in the developing seeds of Arikara Yellow, which darken at harvest and during postharvest storage. In contrast, for CDC Sol and AAC Y073 seeds which remain yellow, phenolic compounds and antioxidant activity were consistently low. Seed brightness (L*) and yellow colour (b*) were negatively correlated with phenolic compounds and antioxidant activity, and conversely seed redness (a*) was positively correlated with phenolic compounds, confirming a negative influence of phenolic compounds on seed coat colour. Yellow bean genotypes had low anthocyanin but were high in β-carotene. Storage temperature influenced condensed tannin and seed coat colour, whereas the duration of storage influenced phenolic compounds, antioxidant activity, and seed coat colour. Higher temperatures (20 or 30 °C) and longer storage duration (120 or 180 d) generally resulted in darker seeds with increasing redness compared with seeds stored at 6 °C or for 60 d. AAC Y073 and CDC Sol with improved seed coat colour may increase consumer preference, value, and marketability of yellow beans.