Battle of Marathon vs Thondia Brown

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Thondia Brown is noticeably darker, more saturated, warmer than Battle of Marathon.

Can I use Thondia Brown instead of Battle of Marathon?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Thondia Brown is noticeably darker, more saturated, warmer than Battle of Marathon.

Can I use Battle of Marathon instead of Thondia Brown?

The same difference applies the other way round: Battle of Marathon is noticeably lighter, 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.