ΔE 16.5, loose: different enough that you should pick on purpose. Greatcoat Grey is much more muted, cooler than Cloudburst Blue. They are different kinds of paint: Army Painter Cloudburst Blue is a one-coat contrast-style paint and Formula P3 Greatcoat Grey is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Greatcoat Grey is much more muted, cooler than Cloudburst Blue. They are different kinds of paint.
The same difference applies the other way round: Cloudburst Blue is much more saturated, 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.
See Cloudburst Blue on the colour wheel See Greatcoat Grey on the colour wheel
All matches: Cloudburst Blue equivalents · Greatcoat Grey equivalents