Deep Azure vs Arachnid Green

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Arachnid Green is noticeably darker, more muted, warmer than Deep Azure.

Can I use Arachnid Green instead of Deep Azure?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Arachnid Green is noticeably darker, more muted, warmer than Deep Azure.

Can I use Deep Azure instead of Arachnid Green?

The same difference applies the other way round: Deep Azure is noticeably lighter, more saturated, cooler.

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.