What colour is sadness?  As it’s referring to weather, I assume grey, but sometimes grey cloud cover can be a reprieve from dazzling heat and sun, and of course, it can bring long-awaited rain.

… so all that day of sad-coloured weather the white cattle went up and down the red field …

Source: Mary Webb, Precious Bane (Harmondsworth: Penguin, 1981), p. 115