High Coverage Yellow vs Phalanx Yellow

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Phalanx Yellow is a little darker, warmer than High Coverage Yellow.

Can I use Phalanx Yellow instead of High Coverage Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Phalanx Yellow is a little darker, warmer than High Coverage Yellow.

Can I use High Coverage Yellow instead of Phalanx Yellow?

The same difference applies the other way round: High Coverage Yellow is a little lighter, cooler.

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.