Lightning Blue vs Arctic Gem

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Arctic Gem is more muted, cooler than Lightning Blue.

Can I use Arctic Gem instead of Lightning Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Arctic Gem is more muted, cooler than Lightning Blue.

Can I use Lightning Blue instead of Arctic Gem?

The same difference applies the other way round: Lightning Blue is more saturated, warmer.

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.