Umbral Umber vs Nocturnal Red

How close are they?

ΔE 27.7, loose: different enough that you should pick on purpose. Nocturnal Red is noticeably lighter, much more saturated, cooler than Umbral Umber.

Can I use Nocturnal Red instead of Umbral Umber?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Nocturnal Red is noticeably lighter, much more saturated, cooler than Umbral Umber.

Can I use Umbral Umber instead of Nocturnal Red?

The same difference applies the other way round: Umbral Umber is noticeably darker, much 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.