Gorthor Brown vs Light Umber

How close are they?

ΔE 9.0, close: similar, with a visible difference side by side. Light Umber is a little darker, cooler than Gorthor Brown.

Can I use Light Umber instead of Gorthor Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Light Umber is a little darker, cooler than Gorthor Brown.

Can I use Gorthor Brown instead of Light Umber?

The same difference applies the other way round: Gorthor Brown is a little lighter, 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.