Is it at all possible that Varian isn't Anduin's father? I'm not going to speculate on hair color since it's entirely possible for Anduin to inherit his mother's hair color. I'm looking deeper towards his affinity for 'the holy light'. Not only is he extremely close to Magni, to the point of considering him an 'uncle' this is a direct quote: "The Holy Light responds well to him, and he feels such a natural affinity to it that both King Magni and High Priest Rohan suggest that Anduin may have a more fitting calling as a Priest."
A natural affinity for the light when neither his parents pursued that path. In reading Tiffin Wrynn's wikipedia I also noticed this: "Though her marriage was arranged at birth in order to give her family a place in the Stormwind House of Nobles she did have a short lived relationship with another man."
This implies that there is AT THE LEAST a chance that Varian isn't Anduin's father.
The other theory is that Anduin is Arthas/Jaina's offspring which could technically be possible but there's a plot-hole why/how Varian would keep this secret or adopt him as his heir. The other man referenced in Tiffin's wiki could also be Arthas but that'd be improbable.