Mutant Hue vs Ivory Tusk

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Ivory Tusk is a little darker, more muted than Mutant Hue.

Can I use Ivory Tusk instead of Mutant Hue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ivory Tusk is a little darker, more muted than Mutant Hue.

Can I use Mutant Hue instead of Ivory Tusk?

The same difference applies the other way round: Mutant Hue is a little lighter, 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.