A colour that has been softened toward grey — less intense than its purest, most vivid version.
Colourwise classifies a colour as muted when HSV saturation is below 40%.
Muted is not the same as pale — a muted colour can still be quite dark (a muted plum), whereas pastel specifically implies high lightness too.
Dusty rose and sage green are muted colours.
'How do I mute a colour?' is one of the most common practical colour-mixing questions — see the mixing guide.