Indian Shadow vs Flat Brown

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Flat Brown is more muted, warmer than Indian Shadow.

Can I use Flat Brown instead of Indian Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Brown is more muted, warmer than Indian Shadow.

Can I use Indian Shadow instead of Flat Brown?

The same difference applies the other way round: Indian Shadow is more saturated, cooler.

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.