ΔE 16.9, loose: different enough that you should pick on purpose. Sky is noticeably lighter, cooler than Wraithbone.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Sky is noticeably lighter, cooler than Wraithbone.
The same difference applies the other way round: Wraithbone is noticeably 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 Wraithbone on the colour wheel See Sky on the colour wheel
All matches: Wraithbone equivalents · Sky equivalents