Underhive Ash vs Viridian Green

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Viridian Green is noticeably darker, cooler than Underhive Ash.

Can I use Viridian Green instead of Underhive Ash?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Viridian Green is noticeably darker, cooler than Underhive Ash.

Can I use Underhive Ash instead of Viridian Green?

The same difference applies the other way round: Underhive Ash is noticeably lighter, 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.