Dithering
Mixing dots of the available colours to give the impression of colours that are not available.
Also: dither
When a device or file can show only a limited number of colours, dithering fills the gaps. Pixels of two or more available colours are scattered together, and from a distance the eye blends them into an in-between shade. Early computer graphics with 16 or 256 colours relied on it heavily, giving them a speckled look. It is still used quietly today to smooth gradients and hide banding, and by inkjet printers, which can only place or not place a drop of each ink.