Ultramarines Blue vs Replenish Blue

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Replenish Blue is a little lighter, cooler than Ultramarines Blue.

Can I use Replenish Blue instead of Ultramarines Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Replenish Blue is a little lighter, cooler than Ultramarines Blue.

Can I use Ultramarines Blue instead of Replenish Blue?

The same difference applies the other way round: Ultramarines Blue is a little darker, 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.