Light Phthalo Green vs Ink White

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Ink White is a little lighter, much more muted than Light Phthalo Green.

Can I use Ink White instead of Light Phthalo Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Ink White is a little lighter, much more muted than Light Phthalo Green.

Can I use Light Phthalo Green instead of Ink White?

The same difference applies the other way round: Light Phthalo Green is a little darker, much 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.