I’ve dyed some cotton fabric yellow with dandelion flowers. I’ve gathered a potful of dandelion flowers, and boiled them for about an hour with some water. Then I’ve removed the flowers, and cooked the fabric again for about an hour in the dyebath. The prewashed, unmordanted, white cotton fabric turned a pretty pale yellow. I’ve exposed the fabric to the sun for some time, and washed it: the color seems to be lightfast and wash fast.