Reference guide The R+L=J theory claims Jon Snow most probably is the son of crown prince Rhaegar Targaryen and Ned's sister Lyanna Stark. The Tower of the Hand has an excellent analysis of this theory: Jon Snow's Parents And Westeros' Citadel also provides a summary: Jon Snow's Parents A Wiki of Ice and Fire: Jon Snow Theories Radio Westeros podcast: A Dragon, a Wolf and a Rose Frequently Asked Questions: How can Jon be a Targaryen if ordinary fire burned his hand? Targaryens are
There's a possible contrast here. Rhaegar in red and black, Lyanna in blue and white. White and black are real life opposites, while red and blue are thematic opposites in ASoIaF, so the grouping of these four colors caught my eye, because they've been grouped elsewhere. Specifically, in Bran and Dany's chapters, except that it's red and white vs. black and blue; e.g., weirwoods vs. the trees the warlocks make shade of the evening from, which has black bark