Longbeard Grey vs Viridian Green

How close are they?

ΔE 20.0, loose: different enough that you should pick on purpose. Viridian Green is noticeably darker, more saturated than Longbeard Grey.

Can I use Viridian Green instead of Longbeard Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Viridian Green is noticeably darker, more saturated than Longbeard Grey.

Can I use Longbeard Grey instead of Viridian Green?

The same difference applies the other way round: Longbeard Grey is noticeably 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.