Muted colours, colours that have been altered in their original value by the addition of others, as opposed to full colours; see mezzotint.
Mixing a full colour with black creates a muted, blackened shade. Alternatively, the painter may muten a colour by mixing the primary colour with its complementary colour. The uniforms of the Askaris pictured above have been muted by dipping them in blackened varnish.
Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909