Fog Grey vs Ultramar Blue

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Ultramar Blue is a little lighter, more saturated than Fog Grey.

Can I use Ultramar Blue instead of Fog Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Ultramar Blue is a little lighter, more saturated than Fog Grey.

Can I use Fog Grey instead of Ultramar Blue?

The same difference applies the other way round: Fog Grey is a little darker, more muted.

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.