Highlord Blue vs Deep Blue

How close are they?

ΔE 27.0, loose: different enough that you should pick on purpose. Deep Blue is a little darker, much more saturated, warmer than Highlord Blue.

Can I use Deep Blue instead of Highlord Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Deep Blue is a little darker, much more saturated, warmer than Highlord Blue.

Can I use Highlord Blue instead of Deep Blue?

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