RAF PRU Blue BS381C/636 vs Regiment Grey

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Regiment Grey is a little darker, more muted than RAF PRU Blue BS381C/636.

Can I use Regiment Grey instead of RAF PRU Blue BS381C/636?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Regiment Grey is a little darker, more muted than RAF PRU Blue BS381C/636.

Can I use RAF PRU Blue BS381C/636 instead of Regiment Grey?

The same difference applies the other way round: RAF PRU Blue BS381C/636 is a little lighter, 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.