Bright Neutral Grey vs Russ. AF Grey N.8

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Russ. AF Grey N.8 is a little darker, more saturated than Bright Neutral Grey.

Can I use Russ. AF Grey N.8 instead of Bright Neutral Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Russ. AF Grey N.8 is a little darker, more saturated than Bright Neutral Grey.

Can I use Bright Neutral Grey instead of Russ. AF Grey N.8?

The same difference applies the other way round: Bright Neutral Grey is a little lighter, more muted.

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.