T'au Light Ochre vs Dark Ochre

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Dark Ochre is a little lighter, much more saturated than T'au Light Ochre.

Can I use Dark Ochre instead of T'au Light Ochre?

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

Can I use T'au Light Ochre instead of Dark Ochre?

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

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.