Regal Blue vs Ultramar Blue

How close are they?

ΔE 15.0, loose: different enough that you should pick on purpose. Ultramar Blue is a little darker, more muted, warmer than Regal Blue.

Can I use Ultramar Blue instead of Regal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Ultramar Blue is a little darker, more muted, warmer than Regal Blue.

Can I use Regal Blue instead of Ultramar Blue?

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