Before I am told that this is a duplicate of this question, know that that question hasn't been answered, and I cant vote or comment on that due to low reputation.
I have read every single rule book and every interpretation of them available online and I cannot find an answer.
In Uno (or other similar games with a challenge aspect. e.g. UNO flip), when someone successfully challenges a Wild Draw 4 (or WD2 or WDC in flip), does play resume on the colour that was asked before the WD4 was played, or does it change to the colour requested by the person who (illegally) played the WD4.
For example, let's say we have some players including Player A and Player B. It is Player A's turn and the colour is red. Player A has quite a few cards, too many to not assume they will be challenged, but Player B is on UNO, and A knows (or deduces) that their last card is red. Player A plays a WD4, calling yellow, gets challenged, and picks up 4 cards. Does play continue on yellow or red?
My logical assumption would be red, since Player A's other benefits of their card were forfeited, the colour obviously should have too, but furthermore, the WD4 was played illegally. If A played a red like they should have, play would be red and B would win.