Rainforest vs Warpstone Glow

How close are they?

ΔE 28.6, loose: different enough that you should pick on purpose. Warpstone Glow is noticeably darker, more muted, cooler than Rainforest.

Can I use Warpstone Glow instead of Rainforest?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Warpstone Glow is noticeably darker, more muted, cooler than Rainforest.

Can I use Rainforest instead of Warpstone Glow?

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