ΔE 14.9, fair: the same colour family, but not a straight swap. Light Phthalo Green is much more saturated than Matt White Primer. They are different kinds of paint: Army Painter Matt White Primer is a technical paint or medium and Golden Light Phthalo Green is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Light Phthalo Green is much more saturated than Matt White Primer. They are different kinds of paint.
The same difference applies the other way round: Matt White Primer is much more muted.
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.
See Matt White Primer on the colour wheel See Light Phthalo Green on the colour wheel
All matches: Matt White Primer equivalents · Light Phthalo Green equivalents