Thunderbird Blue vs Aeldari Emerald

How close are they?

ΔE 20.9, loose: different enough that you should pick on purpose. Aeldari Emerald is noticeably darker, more saturated, warmer than Thunderbird Blue.

Can I use Aeldari Emerald instead of Thunderbird Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Aeldari Emerald is noticeably darker, more saturated, warmer than Thunderbird Blue.

Can I use Thunderbird Blue instead of Aeldari Emerald?

The same difference applies the other way round: Thunderbird Blue is noticeably lighter, more muted, cooler.

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.