Black Green vs Green RLM73

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Green RLM73 is a little lighter, more muted than Black Green.

Can I use Green RLM73 instead of Black Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Green RLM73 is a little lighter, more muted than Black Green.

Can I use Black Green instead of Green RLM73?

The same difference applies the other way round: Black Green 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.