ΔE 27.4, loose: different enough that you should pick on purpose. Hydromiel Yellow is a little lighter, much more saturated, cooler than Skeleton Horde. They are different kinds of paint: Citadel Skeleton Horde is a one-coat contrast-style paint and Green Stuff World Hydromiel Yellow 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 27). Hydromiel Yellow is a little lighter, much more saturated, cooler than Skeleton Horde. They are different kinds of paint.
The same difference applies the other way round: Skeleton Horde is a little darker, much more muted, warmer.
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 Skeleton Horde on the colour wheel See Hydromiel Yellow on the colour wheel
All matches: Skeleton Horde equivalents · Hydromiel Yellow equivalents