ΔE 13.5, fair: the same colour family, but not a straight swap. Phoenix Orange is noticeably lighter, warmer than T'au Light Ochre. They are different kinds of paint: Citadel T'au Light Ochre is an opaque base/layer paint and Green Stuff World Phoenix Orange is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Phoenix Orange is noticeably lighter, warmer than T'au Light Ochre. They are different kinds of paint.
The same difference applies the other way round: T'au Light Ochre is noticeably darker, 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.
See T'au Light Ochre on the colour wheel See Phoenix Orange on the colour wheel
All matches: T'au Light Ochre equivalents · Phoenix Orange equivalents