Regiment Grey vs Hell Bent for Leather

How close are they?

ΔE 8.8, close: similar, with a visible difference side by side. Hell Bent for Leather is a little darker than Regiment Grey.

Can I use Hell Bent for Leather instead of Regiment Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Hell Bent for Leather is a little darker than Regiment Grey.

Can I use Regiment Grey instead of Hell Bent for Leather?

The same difference applies the other way round: Regiment Grey 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.