Lilac vs Fenrisian Grey

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Fenrisian Grey is a little darker, much more muted, cooler than Lilac.

Can I use Fenrisian Grey instead of Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Fenrisian Grey is a little darker, much more muted, cooler than Lilac.

Can I use Lilac instead of Fenrisian Grey?

The same difference applies the other way round: Lilac is a little lighter, much 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.