Fluor Yellow-Orange vs Thick Blood

How close are they?

ΔE 42.4, loose: different enough that you should pick on purpose. Thick Blood is noticeably darker, more muted, cooler than Fluor Yellow-Orange.

Can I use Thick Blood instead of Fluor Yellow-Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Thick Blood is noticeably darker, more muted, cooler than Fluor Yellow-Orange.

Can I use Fluor Yellow-Orange instead of Thick Blood?

The same difference applies the other way round: Fluor Yellow-Orange 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.