Bloodfest Crimson vs Deep Magenta

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Deep Magenta is a little lighter, more saturated, cooler than Bloodfest Crimson.

Can I use Deep Magenta instead of Bloodfest Crimson?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Deep Magenta is a little lighter, more saturated, cooler than Bloodfest Crimson.

Can I use Bloodfest Crimson instead of Deep Magenta?

The same difference applies the other way round: Bloodfest Crimson is a little darker, more muted, 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.