Blue Black vs Royal Purple

How close are they?

ΔE 8.4, close: similar, with a visible difference side by side. Royal Purple is a little darker, more saturated, warmer than Blue Black.

Can I use Royal Purple instead of Blue Black?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Royal Purple is a little darker, more saturated, warmer than Blue Black.

Can I use Blue Black instead of Royal Purple?

The same difference applies the other way round: Blue Black is a little lighter, more muted, cooler.

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.