Guilliman Flesh vs Mummy White

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Mummy White is a little darker, more muted, cooler than Guilliman Flesh.

Can I use Mummy White instead of Guilliman Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Mummy White is a little darker, more muted, cooler than Guilliman Flesh.

Can I use Guilliman Flesh instead of Mummy White?

The same difference applies the other way round: Guilliman Flesh is a little lighter, more saturated, 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.