Thousand Sons Blue vs Ultramarine Blue

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Ultramarine Blue is a little darker than Thousand Sons Blue.

Can I use Ultramarine Blue instead of Thousand Sons Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Ultramarine Blue is a little darker than Thousand Sons Blue.

Can I use Thousand Sons Blue instead of Ultramarine 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.