RAF Middle Stone vs Wasteland Clay

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Wasteland Clay is a little lighter, more muted than RAF Middle Stone.

Can I use Wasteland Clay instead of RAF Middle Stone?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Wasteland Clay is a little lighter, more muted than RAF Middle Stone.

Can I use RAF Middle Stone instead of Wasteland Clay?

The same difference applies the other way round: RAF Middle Stone is a little darker, more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.