Terminatus Stone vs Terran Khaki

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Terran Khaki is a little darker than Terminatus Stone.

Can I use Terran Khaki instead of Terminatus Stone?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Terran Khaki is a little darker than Terminatus Stone.

Can I use Terminatus Stone instead of Terran Khaki?

The same difference applies the other way round: Terminatus Stone is a little lighter.

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.