Ultramar Blue vs Flat Blue

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Flat Blue is more muted, warmer than Ultramar Blue.

Can I use Flat Blue instead of Ultramar Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Blue is more muted, warmer than Ultramar Blue.

Can I use Ultramar Blue instead of Flat Blue?

The same difference applies the other way round: Ultramar Blue is 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.