Regiment Grey vs Faded Purple

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Faded Purple is a little lighter, much more saturated than Regiment Grey.

Can I use Faded Purple instead of Regiment Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Faded Purple is a little lighter, much more saturated than Regiment Grey.

Can I use Regiment Grey instead of Faded Purple?

The same difference applies the other way round: Regiment Grey is a little darker, much 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.