DD also is an extremely good speller. I read somewhere, possibly not somewhere reliable, that this is associated with synesthesia.
The funny thing about this for me is that I am totally fascinated with it and DD seems to feel it's sort of ho-hum. Like, okay, yes, of course Tuesday is blue. What's the big deal?
LOL

I just asked my kids:
DS8, awful speller, said that words are small black letters on a giant white space (um, ok).
DD10, very artistic (drawing at the moment) and a crackerjack speller, said that 7 is brown, 4 is green, and the colours of the numbers helped her figure out that 7+7=14 when she was younger. (Chatty DS interrupted her at that point, but I have a feeling she had more to say).
Interesting... So of course now I'm wondering... (lol) How do I get my DS to start seeing more colours and maybe improve his spelling? heh heh