Fluorescent Red vs Thick Blood

How close are they?

ΔE 48.5, loose: different enough that you should pick on purpose. Thick Blood is noticeably darker, much more muted than Fluorescent Red.

Can I use Thick Blood instead of Fluorescent Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 49). Thick Blood is noticeably darker, much more muted than Fluorescent Red.

Can I use Fluorescent Red instead of Thick Blood?

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

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.