Dorn Yellow vs Primary Yellow

How close are they?

ΔE 17.2, loose: different enough that you should pick on purpose. Primary Yellow is noticeably lighter, more saturated, cooler than Dorn Yellow.

Can I use Primary Yellow instead of Dorn Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Primary Yellow is noticeably lighter, more saturated, cooler than Dorn Yellow.

Can I use Dorn Yellow instead of Primary Yellow?

The same difference applies the other way round: Dorn Yellow is noticeably darker, more muted, warmer.

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.