Light Phthalo Green vs Russ. AF Light Blue

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Russ. AF Light Blue is noticeably darker, more muted than Light Phthalo Green.

Can I use Russ. AF Light Blue instead of Light Phthalo Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Russ. AF Light Blue is noticeably darker, more muted than Light Phthalo Green.

Can I use Light Phthalo Green instead of Russ. AF Light Blue?

The same difference applies the other way round: Light Phthalo Green is noticeably lighter, more saturated.

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.