Dark Green Blue vs Forest Green

How close are they?

ΔE 29.0, loose: different enough that you should pick on purpose. Forest Green is much more saturated, warmer than Dark Green Blue.

Can I use Forest Green instead of Dark Green Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Forest Green is much more saturated, warmer than Dark Green Blue.

Can I use Dark Green Blue instead of Forest Green?

The same difference applies the other way round: Dark Green Blue is much more muted, 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.