Carnage Red vs Red RLM23

How close are they?

ΔE 22.9, loose: different enough that you should pick on purpose. Red RLM23 is noticeably lighter, much more saturated than Carnage Red.

Can I use Red RLM23 instead of Carnage Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Red RLM23 is noticeably lighter, much more saturated than Carnage Red.

Can I use Carnage Red instead of Red RLM23?

The same difference applies the other way round: Carnage Red 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.