ΔE 15.8, loose: different enough that you should pick on purpose. Blood Tracker Brown is noticeably darker than T'au Light Ochre.
Can I use Blood Tracker Brown instead of T'au Light Ochre?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Blood Tracker Brown is noticeably darker than T'au Light Ochre.
Can I use T'au Light Ochre instead of Blood Tracker Brown?
The same difference applies the other way round: T'au Light Ochre is noticeably lighter.
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.