Why use one colour...?

A wash of red and ochres on the roof with drips of blue, purple and yellow green

Another wide range of colours to represent the ancient knapped flint

Here are the first two stages of painting the flint house. As you can see, one colour will never do!!
When the ochres and purples on the roof were still damp I dropped in some bright and quite opaque yellows and greens for the lychen.
For the flints, I can never decide on their colour, they are warm yet cool at the same time. So I had fun with three puddles of colours, a yellow ochre, a purple and a pink, then I dropped in some extras here and there mostly blues.
In the first image you can see the spots of blue mask that I had splashed around. Removing this once the wall was painted left me with some bright white flashes for the flint. I can always dull these down with another wash later and some shadows, if necessary.
Sky next!! It will be a greeny blue, to contrast all the purples.