ΔE 15.2, loose: different enough that you should pick on purpose. Turquoise Ink is a little lighter, much more saturated than Nuln Oil. They are different kinds of paint: Citadel Nuln Oil is a wash/shade and Nocturna Models Turquoise Ink is an ink, glaze or transparent 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 15). Turquoise Ink is a little lighter, much more saturated than Nuln Oil. They are different kinds of paint.
The same difference applies the other way round: Nuln Oil is a little darker, much more muted.
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 Nuln Oil on the colour wheel See Turquoise Ink on the colour wheel
All matches: Nuln Oil equivalents · Turquoise Ink equivalents