Bright Pale Green vs Flanker Light Blue

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Flanker Light Blue is a little darker, more muted, cooler than Bright Pale Green.

Can I use Flanker Light Blue instead of Bright Pale Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Flanker Light Blue is a little darker, more muted, cooler than Bright Pale Green.

Can I use Bright Pale Green instead of Flanker Light Blue?

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