Diabolic Plum vs Cygnar Blue Base

How close are they?

ΔE 25.9, loose: different enough that you should pick on purpose. Cygnar Blue Base is a little lighter, more saturated, cooler than Diabolic Plum.

Can I use Cygnar Blue Base instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Cygnar Blue Base is a little lighter, more saturated, cooler than Diabolic Plum.

Can I use Diabolic Plum instead of Cygnar Blue Base?

The same difference applies the other way round: Diabolic Plum 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.