Deep Blue vs Alien Purple

How close are they?

ΔE 17.0, loose: different enough that you should pick on purpose. Alien Purple is more muted, warmer than Deep Blue.

Can I use Alien Purple instead of Deep Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Alien Purple is more muted, warmer than Deep Blue.

Can I use Deep Blue instead of Alien Purple?

The same difference applies the other way round: Deep Blue is more saturated, 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.