ΔE 13.8, fair: the same colour family, but not a straight swap. Blue Ink is noticeably darker, more saturated, warmer than Thunderhawk Blue. They are different kinds of paint: Citadel Thunderhawk Blue is an opaque base/layer paint and Reaper Blue Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Blue Ink is noticeably darker, more saturated, warmer than Thunderhawk Blue. They are different kinds of paint.
The same difference applies the other way round: Thunderhawk Blue is noticeably lighter, more muted, 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 Thunderhawk Blue on the colour wheel See Blue Ink on the colour wheel
All matches: Thunderhawk Blue equivalents · Blue Ink equivalents