Thar Brown vs IAF Sand

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. IAF Sand is more muted, cooler than Thar Brown.

Can I use IAF Sand instead of Thar Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). IAF Sand is more muted, cooler than Thar Brown.

Can I use Thar Brown instead of IAF Sand?

The same difference applies the other way round: Thar Brown 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.