Dark blue vs blue-grey eyes

Dark blue reads as deep and saturated, often with a strong limbal ring that makes the iris look sharply outlined. Blue-grey reads as pale and desaturated. The two are rarely confused in good light; the confusion happens in dim conditions, where dark blue loses its depth and blue-grey loses its blue, and both converge on a similar muted appearance.

Typical distribution

Dark blue frequently shows a marked contrast between a lighter centre and a much darker outer edge; blue-grey is usually more uniform.

Undertone

Both are cool. The difference is depth and saturation rather than temperature — dark blue is considerably deeper and carries more colour.

Central or outer ring

A strong dark limbal ring is much more characteristic of dark blue and is one of the more useful distinguishing features between these two.

Lighting effects

In low light dark blue can read almost neutral-dark and blue-grey almost plain grey, which is when the two look most alike. In daylight they are clearly different.