Bright Green Blue vs Foul Green

How close are they?

ΔE 23.2, loose: different enough that you should pick on purpose. Foul Green is a little darker, much more saturated than Bright Green Blue.

Can I use Foul Green instead of Bright Green Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Foul Green is a little darker, much more saturated than Bright Green Blue.

Can I use Bright Green Blue instead of Foul Green?

The same difference applies the other way round: Bright Green Blue is a little lighter, much 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.