Old Red vs Fulgrim Pink

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Fulgrim Pink is a little lighter, cooler than Old Red.

Can I use Fulgrim Pink instead of Old Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Fulgrim Pink is a little lighter, cooler than Old Red.

Can I use Old Red instead of Fulgrim Pink?

The same difference applies the other way round: Old Red is a little darker, 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.