Rock Grey vs Green Yellow

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Green Yellow is more muted, warmer than Rock Grey.

Can I use Green Yellow instead of Rock Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Green Yellow is more muted, warmer than Rock Grey.

Can I use Rock Grey instead of Green Yellow?

The same difference applies the other way round: Rock Grey is more saturated, cooler.

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.