Those identifying shafaq with redness appeal to the familiar post-sunset red glow and narrations describing prayer times; Abu Yusuf and Muhammad use this marker for Maghrib’s ending.
Those identifying it with the later whiteness take the residual light after redness as the relevant final twilight marker; in Abu Hanifah’s view, Maghrib continues until that light disappears.
The disagreement concerns the identity of shafaq, not the number or status of the prayers.