Thondia Brown vs Dark Umber

How close are they?

ΔE 16.9, loose: different enough that you should pick on purpose. Dark Umber is noticeably darker, more muted than Thondia Brown.

Can I use Dark Umber instead of Thondia Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Dark Umber is noticeably darker, more muted than Thondia Brown.

Can I use Thondia Brown instead of Dark Umber?

The same difference applies the other way round: Thondia Brown is noticeably lighter, 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.