Gradient
A smooth, gradual change from one colour to another across an area.
Also: colour ramp, blend, ombré
A gradient blends colours step by step so finely that no edge is seen. It may run in a straight line, spread out from a centre or sweep round a point. Skies at dusk are natural gradients. On screens they are used for backgrounds, buttons and charts. The route between the two colours matters: a blend calculated carelessly can pass through a dull grey in the middle, while one calculated in a colour space designed around human vision stays lively. In textiles and hair colouring the same effect is called ombré.