Pastel Yellow vs Sulfur Yellow

How close are they?

ΔE 22.9, loose: different enough that you should pick on purpose. Sulfur Yellow is a little darker, much more saturated, warmer than Pastel Yellow.

Can I use Sulfur Yellow instead of Pastel Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Sulfur Yellow is a little darker, much more saturated, warmer than Pastel Yellow.

Can I use Pastel Yellow instead of Sulfur Yellow?

The same difference applies the other way round: Pastel Yellow is a little lighter, much more muted, 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.