Russian Dark Brown 6K vs Thondia Brown

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Thondia Brown is noticeably lighter, more saturated, warmer than Russian Dark Brown 6K.

Can I use Thondia Brown instead of Russian Dark Brown 6K?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Thondia Brown is noticeably lighter, more saturated, warmer than Russian Dark Brown 6K.

Can I use Russian Dark Brown 6K instead of Thondia Brown?

The same difference applies the other way round: Russian Dark Brown 6K is noticeably darker, more muted, cooler.

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.