Mayhem Red vs Crimson

How close are they?

ΔE 28.6, loose: different enough that you should pick on purpose. Crimson is noticeably lighter, more saturated, cooler than Mayhem Red.

Can I use Crimson instead of Mayhem Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Crimson is noticeably lighter, more saturated, cooler than Mayhem Red.

Can I use Mayhem Red instead of Crimson?

The same difference applies the other way round: Mayhem Red is noticeably darker, more muted, 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.