ΔE 17.2, loose: different enough that you should pick on purpose. Primary Cyan is noticeably lighter, more saturated than Lothern Blue. They are different kinds of paint: Citadel Lothern Blue is an opaque base/layer paint and Golden Primary Cyan 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 17). Primary Cyan is noticeably lighter, more saturated than Lothern Blue. They are different kinds of paint.
The same difference applies the other way round: Lothern Blue is noticeably darker, 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 Lothern Blue on the colour wheel See Primary Cyan on the colour wheel
All matches: Lothern Blue equivalents · Primary Cyan equivalents