ΔE 6.6, close: similar, with a visible difference side by side. Flat Blue is a little lighter than Cerulean Blue Deep.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Flat Blue is a little lighter than Cerulean Blue Deep.
The same difference applies the other way round: Cerulean Blue Deep is a little darker.
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.
See Cerulean Blue Deep on the colour wheel See Flat Blue on the colour wheel
All matches: Cerulean Blue Deep equivalents · Flat Blue equivalents