Mephiston Red vs Sanguine Base

How close are they?

ΔE 21.9, loose: different enough that you should pick on purpose. Sanguine Base is noticeably darker, much more muted than Mephiston Red.

Can I use Sanguine Base instead of Mephiston Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Sanguine Base is noticeably darker, much more muted than Mephiston Red.

Can I use Mephiston Red instead of Sanguine Base?

The same difference applies the other way round: Mephiston Red is noticeably lighter, much 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.