Phoenix Flames 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 Phoenix Flames.

Can I use Rucksack Tan instead of Phoenix Flames?

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

Can I use Phoenix Flames instead of Rucksack Tan?

The same difference applies the other way round: Phoenix Flames 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.