Inner Light vs Rucksack Tan

How close are they?

ΔE 25.6, loose: different enough that you should pick on purpose. Rucksack Tan is noticeably darker, more muted, cooler than Inner Light.

Can I use Rucksack Tan instead of Inner Light?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Rucksack Tan is noticeably darker, more muted, cooler than Inner Light.

Can I use Inner Light instead of Rucksack Tan?

The same difference applies the other way round: Inner Light is noticeably lighter, more saturated, 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.