Zealot Yellow vs Estus Yellow

How close are they?

ΔE 28.5, loose: different enough that you should pick on purpose. Estus Yellow is noticeably lighter, much more muted, cooler than Zealot Yellow.

Can I use Estus Yellow instead of Zealot Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Estus Yellow is noticeably lighter, much more muted, cooler than Zealot Yellow.

Can I use Zealot Yellow instead of Estus Yellow?

The same difference applies the other way round: Zealot Yellow is noticeably darker, much 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.