Yellow Ochre vs Rucksack Tan

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Rucksack Tan is much more saturated, cooler than Yellow Ochre.

Can I use Rucksack Tan instead of Yellow Ochre?

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

Can I use Yellow Ochre instead of Rucksack Tan?

The same difference applies the other way round: Yellow Ochre is 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.