Light Blue vs Foul Green

How close are they?

ΔE 28.4, loose: different enough that you should pick on purpose. Foul Green is a little darker, much more saturated, warmer than Light Blue.

Can I use Foul Green instead of Light Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Foul Green is a little darker, much more saturated, warmer than Light Blue.

Can I use Light Blue instead of Foul Green?

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