gradient
On Danbooru, this refers to a color gradient, which is a smooth, gradual shift from one color to another across an image, based on the position of a pixel relative to a defined point or line.
An image may still be considered monochrome if it just applies a gradient across what would otherwise be a black & white image, even if the gradient has a multitude of colors, e.g. Danbooru post #61761 or Danbooru post #141581.