""Asking for evidence to back up an assertion is not out of line in most instances.""

I suspect it may not be out of line. However, if a question is asked, and the person already knows there is no answer available, what's the point and goal? It starts a zero game, with little possible gain for anyone.

In my example, "I prefer the colour blue over yellow"", to stimulate meaningful two - way discussion, an appropriate question may be, why do you feel that way (versus, where is your evidence that blue is better than yellow).

Why would one poster ask for such evidence (aka scientific evidence) if they know the other poster does not have the type of evidence that will satisfy their question? IMO, what stimulates that question is more revealing in these situations.