Incubi Darkness vs Field Gray 1

How close are they?

ΔE 9.2, close: similar, with a visible difference side by side. Field Gray 1 is a little lighter, more muted than Incubi Darkness.

Can I use Field Gray 1 instead of Incubi Darkness?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Field Gray 1 is a little lighter, more muted than Incubi Darkness.

Can I use Incubi Darkness instead of Field Gray 1?

The same difference applies the other way round: Incubi Darkness is a little darker, more saturated.

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.