Thunderbird Blue vs Akhelian Green

How close are they?

ΔE 24.1, loose: different enough that you should pick on purpose. Akhelian Green is noticeably darker, more muted, cooler than Thunderbird Blue.

Can I use Akhelian Green instead of Thunderbird Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Akhelian Green is noticeably darker, more muted, cooler than Thunderbird Blue.

Can I use Thunderbird Blue instead of Akhelian Green?

The same difference applies the other way round: Thunderbird Blue 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.