Permanent Yellow vs Light Yellow

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Light Yellow is a little darker, much more muted than Permanent Yellow.

Can I use Light Yellow instead of Permanent Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Light Yellow is a little darker, much more muted than Permanent Yellow.

Can I use Permanent Yellow instead of Light Yellow?

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