Rucksack Tan vs Sand Yellow

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Sand Yellow is noticeably lighter, more saturated than Rucksack Tan.

Can I use Sand Yellow instead of Rucksack Tan?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Sand Yellow is noticeably lighter, more saturated than Rucksack Tan.

Can I use Rucksack Tan instead of Sand Yellow?

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

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.