Gladeshard vs Light Green (Blue Shade)

How close are they?

ΔE 22.9, loose: different enough that you should pick on purpose. Light Green (Blue Shade) is a little lighter, cooler than Gladeshard.

Can I use Light Green (Blue Shade) instead of Gladeshard?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Light Green (Blue Shade) is a little lighter, cooler than Gladeshard.

Can I use Gladeshard instead of Light Green (Blue Shade)?

The same difference applies the other way round: Gladeshard is a little darker, 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.