Abaddon Black vs Royal Purple

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Royal Purple is a little lighter, much more saturated than Abaddon Black.

Can I use Royal Purple instead of Abaddon Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Royal Purple is a little lighter, much more saturated than Abaddon Black.

Can I use Abaddon Black instead of Royal Purple?

The same difference applies the other way round: Abaddon Black is a little darker, much 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.