saffron or safflower
Saffron is the dried stigmas of a crocus, both aromatic and colouring. Safflower gives dried petals that mostly colour food yellow and taste of very little.
Saffron is sold in small amounts in the spice aisle of supermarkets or from spice shops; safflower petals are easier to find in Turkish or Eastern European grocers, from online spice sellers or in pharmacies. In Hungarian cooking safflower is often used as a cheaper stand-in for saffron, mainly to turn broths golden.
Substitute:
The two can stand in for each other where only colour matters, but safflower lacks saffron's bitter, floral aroma. A pinch of turmeric also colours yellow, but it tastes earthy and becomes noticeable in larger amounts.