ΔE 14.2, fair: the same colour family, but not a straight swap. Grey Blue is a little lighter, more muted, warmer than Flat Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Grey Blue is a little lighter, more muted, warmer than Flat Blue.
The same difference applies the other way round: Flat Blue is a little darker, more saturated, 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.
See Flat Blue on the colour wheel See Grey Blue on the colour wheel
All matches: Flat Blue equivalents · Grey Blue equivalents