Broken colour.

Broken Colour: "'Broken' colour refers to the subtractive combination of contrasting colours: the individual intensity of two or more brightly coloured paints is broken or dulled by combining them in mixtures...
...colours used 'pure' elsewhere in the composition are combined to give broken grey variants. Retaining the liverly qualities of the original bright colours, these ensure the picture's colouristic unity while permitting a painterly economy of means during rapid work en plein air...
...The key to making coloured greys is including both warm and cool colours in the mixture; adding a touch of red to a blue-green mixture is the easiest, most effective, way to 'break' it and render it greyish. The further apart the colours on on the colour circle, the more broken, or grey, will be their colour when combined."


(Quote source: The Art of Impressionism: Painting technique and the making of modernity by Anthea Callen. Yale University Press. p150)