T'au Light Ochre vs Sandy Tan

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Sandy Tan is a little lighter, cooler than T'au Light Ochre.

Can I use Sandy Tan instead of T'au Light Ochre?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Sandy Tan is a little lighter, cooler than T'au Light Ochre.

Can I use T'au Light Ochre instead of Sandy Tan?

The same difference applies the other way round: T'au Light Ochre is a little darker, 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.