Deep Amethyst vs Blue

How close are they?

ΔE 9.1, close: similar, with a visible difference side by side. Blue is a little lighter, more muted than Deep Amethyst.

Can I use Blue instead of Deep Amethyst?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Blue is a little lighter, more muted than Deep Amethyst.

Can I use Deep Amethyst instead of Blue?

The same difference applies the other way round: Deep Amethyst is a little darker, more saturated.

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.