Guilliman Flesh vs Bright Slate Grey

How close are they?

ΔE 39.7, loose: different enough that you should pick on purpose. Bright Slate Grey is a little darker, much more muted than Guilliman Flesh.

Can I use Bright Slate Grey instead of Guilliman Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 40). Bright Slate Grey is a little darker, much more muted than Guilliman Flesh.

Can I use Guilliman Flesh instead of Bright Slate Grey?

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