Fallen Grass vs Light Green Blue

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Light Green Blue is noticeably lighter, more muted, cooler than Fallen Grass.

Can I use Light Green Blue instead of Fallen Grass?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Light Green Blue is noticeably lighter, more muted, cooler than Fallen Grass.

Can I use Fallen Grass instead of Light Green Blue?

The same difference applies the other way round: Fallen Grass is noticeably darker, 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.