Gradient.

A smooth change of colour across an area. On white-ink processes the question is what it fades into. Colour to colour is safe: Inkthreadable says gradients work if they are from colour to colour, because every pixel stays opaque. A fade into transparency is not, because the white underneath does not fade with it. On a dark shirt Printify says the underbase shows through and the gradient looks uneven or patchy, and it recommends a halftone gradient of small opaque dots instead; Printful's DTF guide rules out gradients from solid to transparent. Sublimation has no white layer, and Redbubble rates every kind of fade as OK on it while marking colour or white fading to nothing as tricky on DTG. The same file can therefore be right for a mug and wrong for a black hoodie.