T'au Light Ochre vs Nickel Azo Yellow

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Nickel Azo Yellow is a little lighter, more saturated, cooler than T'au Light Ochre.

Can I use Nickel Azo Yellow instead of T'au Light Ochre?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Nickel Azo Yellow is a little lighter, more saturated, cooler than T'au Light Ochre.

Can I use T'au Light Ochre instead of Nickel Azo Yellow?

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