T'au Light Ochre vs Dark Sun Yellow

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Dark Sun Yellow is more muted than T'au Light Ochre.

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

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dark Sun Yellow is more muted than T'au Light Ochre.

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

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

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.