Brethil Blush vs Flat Flesh 2

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Flat Flesh 2 is noticeably lighter, more muted than Brethil Blush.

Can I use Flat Flesh 2 instead of Brethil Blush?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Flat Flesh 2 is noticeably lighter, more muted than Brethil Blush.

Can I use Brethil Blush instead of Flat Flesh 2?

The same difference applies the other way round: Brethil Blush is noticeably darker, 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.