I think he's saying that in the fic he wrote him as taller and with a darker hair color because he wanted the hair color to contrast with Matthew Lewis', and Matthew Lewis would look stupid with lighter hair.

Which is stupid to begin with, and also such a strange, obfuscated way to phrase it. I'm thinking that Andy thinks he's sounding conversationally Irish, but really he just sounds confusing.

Weaver of word magic, indeed.