Warp Lightning vs Mummy White

How close are they?

ΔE 54.5, loose: different enough that you should pick on purpose. Mummy White is a little darker, much more muted, warmer than Warp Lightning.

Can I use Mummy White instead of Warp Lightning?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 55). Mummy White is a little darker, much more muted, warmer than Warp Lightning.

Can I use Warp Lightning instead of Mummy White?

The same difference applies the other way round: Warp Lightning is a little lighter, much more saturated, cooler.

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.