Mutant Hue vs Morrow White

How close are they?

ΔE 22.0, loose: different enough that you should pick on purpose. Morrow White is noticeably lighter, much more muted than Mutant Hue.

Can I use Morrow White instead of Mutant Hue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Morrow White is noticeably lighter, much more muted than Mutant Hue.

Can I use Mutant Hue instead of Morrow White?

The same difference applies the other way round: Mutant Hue is noticeably darker, much more saturated.

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.