Thousand Sons Blue vs Brilliant Blue

How close are they?

ΔE 6.0, close: similar, with a visible difference side by side. Brilliant Blue is a little darker than Thousand Sons Blue.

Can I use Brilliant Blue instead of Thousand Sons Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Brilliant Blue is a little darker than Thousand Sons Blue.

Can I use Thousand Sons Blue instead of Brilliant Blue?

The same difference applies the other way round: Thousand Sons Blue is a little 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.