Highlord Blue vs Mystic Blue

How close are they?

ΔE 9.4, close: similar, with a visible difference side by side. Mystic Blue is a little darker than Highlord Blue.

Can I use Mystic Blue instead of Highlord Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mystic Blue is a little darker than Highlord Blue.

Can I use Highlord Blue instead of Mystic Blue?

The same difference applies the other way round: Highlord Blue is a little lighter.

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.