ΔE 17.3, loose: different enough that you should pick on purpose. Speed Yellow is a little lighter, much more saturated than Dorn Yellow. They are different kinds of paint: Citadel Dorn Yellow is an opaque base/layer paint and Scale75 Speed Yellow is a fluorescent or effect paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Speed Yellow is a little lighter, much more saturated than Dorn Yellow. They are different kinds of paint.
The same difference applies the other way round: Dorn Yellow is a little darker, 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 Dorn Yellow on the colour wheel See Speed Yellow on the colour wheel
All matches: Dorn Yellow equivalents · Speed Yellow equivalents