Fluor Orange vs Thick Blood

How close are they?

ΔE 40.5, loose: different enough that you should pick on purpose. Thick Blood is noticeably darker, much more muted, cooler than Fluor Orange.

Can I use Thick Blood instead of Fluor Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 41). Thick Blood is noticeably darker, much more muted, cooler than Fluor Orange.

Can I use Fluor Orange instead of Thick Blood?

The same difference applies the other way round: Fluor Orange is noticeably lighter, 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.