Thousand Sons Blue vs Blue Base

How close are they?

ΔE 9.0, close: similar, with a visible difference side by side. Blue Base is warmer than Thousand Sons Blue.

Can I use Blue Base instead of Thousand Sons Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Blue Base is warmer than Thousand Sons Blue.

Can I use Thousand Sons Blue instead of Blue Base?

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