ΔE 9.8, close: similar, with a visible difference side by side. Screaming Skull is a little darker, more saturated, cooler than Bony Spikes.
✓ Army Painter Fanatic conversion chart lists these two as equivalents.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Screaming Skull is a little darker, more saturated, cooler than Bony Spikes.
The same difference applies the other way round: Bony Spikes is a little lighter, 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 Bony Spikes on the colour wheel See Screaming Skull on the colour wheel
All matches: Bony Spikes equivalents · Screaming Skull equivalents