hey stein_free,
wow you have a fat cola growing there...
...so i'm about 4 and a half weeks into flowering, and i noticed my fan leaves are starting to yellow pretty badly too. i believe this is normal, as you're not supposed to give the plants any nitrogen. i've read that the fan leaves are the storage facilities for the plants...they keep sugars and whatever else inside the fan leaves (which is why you ought not to cut off the fan leaves in favor of the bud leaves). so, if you're late into flowering, it's probably the plant just dipping deep into its reserves for that last little bit before harvest.
hope this helps. i don't believe you have anything to worry about. your plants look great!