Black Templar vs Muddy Ground

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Muddy Ground is a little lighter, more saturated than Black Templar.

Can I use Muddy Ground instead of Black Templar?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Muddy Ground is a little lighter, more saturated than Black Templar.

Can I use Black Templar instead of Muddy Ground?

The same difference applies the other way round: Black Templar is a little darker, more muted.

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.