ΔE 17.4, loose: different enough that you should pick on purpose. Cobalt Turquois is noticeably darker, warmer than Nighthaunt Gloom. They are different kinds of paint: Citadel Nighthaunt Gloom is a one-coat contrast-style paint and Golden Cobalt Turquois 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). Cobalt Turquois is noticeably darker, warmer than Nighthaunt Gloom. They are different kinds of paint.
The same difference applies the other way round: Nighthaunt Gloom is noticeably lighter, 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 Nighthaunt Gloom on the colour wheel See Cobalt Turquois on the colour wheel
All matches: Nighthaunt Gloom equivalents · Cobalt Turquois equivalents