Why does the inside of fried chicken sometimes turn brown?
The darkening is harmless.  It is more likely to occur in younger birds, since their bones are immature, soft, and porous.  It is also exacerbated by deep chilling or freezing, since microscopic ice crystallization will allow various colored substances (e.g., marrow) to coalesce around the bones.  When the meat is cooked, these substances will usually turn brown.

Therefore, interior browning can be avoided by using older chickens and avoiding meat that has ever been frozen.  Given that most commercial chickens in large countries like the USA are slaughtered very young (less than seven weeks old), it may be difficult, but not impossible, to find an older chicken.