Umber Ink vs Ink Black

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Ink Black is a little darker, more muted than Umber Ink.

Can I use Ink Black instead of Umber Ink?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Ink Black is a little darker, more muted than Umber Ink.

Can I use Umber Ink instead of Ink Black?

The same difference applies the other way round: Umber Ink is a little lighter, 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.