Brown Grey Wash vs Red Wash

How close are they?

ΔE 49.7, loose: different enough that you should pick on purpose. Red Wash is noticeably lighter, much more saturated than Brown Grey Wash.

Can I use Red Wash instead of Brown Grey Wash?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 50). Red Wash is noticeably lighter, much more saturated than Brown Grey Wash.

Can I use Brown Grey Wash instead of Red Wash?

The same difference applies the other way round: Brown Grey Wash is noticeably 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.