Magnus the Red vs Warlord Purple

How close are they?

ΔE 20.2, loose: different enough that you should pick on purpose. Warlord Purple is noticeably lighter, more saturated than Magnus the Red.

Can I use Warlord Purple instead of Magnus the Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Warlord Purple is noticeably lighter, more saturated than Magnus the Red.

Can I use Magnus the Red instead of Warlord Purple?

The same difference applies the other way round: Magnus the Red is noticeably darker, more muted.

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.