Dark Umber vs Dark Red

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Dark Red is noticeably lighter, more saturated, cooler than Dark Umber.

Can I use Dark Red instead of Dark Umber?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Dark Red is noticeably lighter, more saturated, cooler than Dark Umber.

Can I use Dark Umber instead of Dark Red?

The same difference applies the other way round: Dark Umber 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.