Magnus the Red vs Royal Purple

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Royal Purple is more muted, cooler than Magnus the Red.

Can I use Royal Purple instead of Magnus the Red?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Royal Purple is more muted, cooler than Magnus the Red.

Can I use Magnus the Red instead of Royal Purple?

The same difference applies the other way round: Magnus the Red is 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.