Wow, you are correct about Radiant being blonde. I won’t surrender my take on it just yet
Let’s just pivot this theory kool-aid to Shallan’s mum, a herald, being of the Royal line from Nalthis and being able to change her hair color at will.
Let’s drink deeper and theorize that her mum is actually the daughter of a certain Scholar present on Roshar and that Shallan has a grand father roaming around that she hasn’t bumped shoulders with yet
Take another sip of the kool-aid, the Scholar’s daughter being a herald would provide one explanation why he returned to Roshar to aid in Gavilar’s research. He could be motivated to find a way to free his daughter
And let’s be real - wouldn’t it be awesome to pair Shallan and the Scholar on a journey? Why haven’t we had this combo yet?