Solar Yellow vs T'au Light Ochre

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. T'au Light Ochre is cooler than Solar Yellow.

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

Usually, if you are not matching paint already on the model. Close match (ΔE 6). T'au Light Ochre is cooler than Solar Yellow.

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

The same difference applies the other way round: Solar Yellow is 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.