WWI German Mauve vs Deep Grey

How close are they?

ΔE 17.0, loose: different enough that you should pick on purpose. Deep Grey is noticeably darker, more muted, cooler than WWI German Mauve.

Can I use Deep Grey instead of WWI German Mauve?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Deep Grey is noticeably darker, more muted, cooler than WWI German Mauve.

Can I use WWI German Mauve instead of Deep Grey?

The same difference applies the other way round: WWI German Mauve is noticeably lighter, more saturated, warmer.

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.