Warp Lightning vs Estus Yellow

How close are they?

ΔE 36.3, loose: different enough that you should pick on purpose. Estus Yellow is noticeably lighter, warmer than Warp Lightning.

Can I use Estus Yellow instead of Warp Lightning?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Estus Yellow is noticeably lighter, warmer than Warp Lightning.

Can I use Warp Lightning instead of Estus Yellow?

The same difference applies the other way round: Warp Lightning is noticeably darker, 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.