Neon Yellow vs Thick Blood

How close are they?

ΔE 102.8, loose: different enough that you should pick on purpose. Thick Blood is noticeably darker, much more muted, warmer than Neon Yellow.

Can I use Thick Blood instead of Neon Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 103). Thick Blood is noticeably darker, much more muted, warmer than Neon Yellow.

Can I use Neon Yellow instead of Thick Blood?

The same difference applies the other way round: Neon Yellow is noticeably lighter, much more saturated, 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.