Morghast Bone vs Hykey Yellow

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Hykey Yellow is noticeably lighter, much more saturated than Morghast Bone.

Can I use Hykey Yellow instead of Morghast Bone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Hykey Yellow is noticeably lighter, much more saturated than Morghast Bone.

Can I use Morghast Bone instead of Hykey Yellow?

The same difference applies the other way round: Morghast Bone is noticeably darker, much more muted.

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.