T'au Light Ochre vs Ember Orange

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Ember Orange is much more saturated, warmer than T'au Light Ochre.

Can I use Ember Orange instead of T'au Light Ochre?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ember Orange is much more saturated, warmer than T'au Light Ochre.

Can I use T'au Light Ochre instead of Ember Orange?

The same difference applies the other way round: T'au Light Ochre is much more muted, 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.