Karak Stone vs Rucksack Tan

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Rucksack Tan is a little darker, much more saturated, cooler than Karak Stone.

Can I use Rucksack Tan instead of Karak Stone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Rucksack Tan is a little darker, much more saturated, cooler than Karak Stone.

Can I use Karak Stone instead of Rucksack Tan?

The same difference applies the other way round: Karak Stone is a little lighter, much 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.