ΔE 10.2, fair: the same colour family, but not a straight swap. Scaly Highlight is a little lighter, cooler than Aeldari Emerald. They are different kinds of paint: Citadel Aeldari Emerald is a one-coat contrast-style paint and Reaper Scaly Highlight is an opaque base/layer 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 10). Scaly Highlight is a little lighter, cooler than Aeldari Emerald. They are different kinds of paint.
The same difference applies the other way round: Aeldari Emerald is a little darker, 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 Aeldari Emerald on the colour wheel See Scaly Highlight on the colour wheel
All matches: Aeldari Emerald equivalents · Scaly Highlight equivalents