Rucksack Tan vs Plague Brown

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Plague Brown is noticeably lighter, much more saturated, warmer than Rucksack Tan.

Can I use Plague Brown instead of Rucksack Tan?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Plague Brown is noticeably lighter, much more saturated, warmer than Rucksack Tan.

Can I use Rucksack Tan instead of Plague Brown?

The same difference applies the other way round: Rucksack Tan is noticeably darker, much 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.