Royal Blue vs Light Ultramarine

How close are they?

ΔE 28.2, loose: different enough that you should pick on purpose. Light Ultramarine is a little darker, much more saturated, warmer than Royal Blue.

Can I use Light Ultramarine instead of Royal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Light Ultramarine is a little darker, much more saturated, warmer than Royal Blue.

Can I use Royal Blue instead of Light Ultramarine?

The same difference applies the other way round: Royal 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.