Hello: If this has happened ONLY ONCE I would not be concerned.
Chickens, especially hens produce what is called Cecal droppings. Every once in a while they will poop something that is not normal
The white is actually the urate part of the poop. Birds do not have a bladder so they incorporate a whitle urate in with the poop. The yellow is a bit of a worry, but like I said if this is rare for her, then do not worry. I would Isolate her and feed her and water her seperately. Any sort of a disease in a bird can be reconizec by what is going on with the poop color and consistancy.
Next time she poops inform us once again what it looks like. You are very dilegent in your care of your birds. watching poop color and consistancy is a great way to keep an eye on the health of your birds. I have feed sacks down were some of my birds roost and night. I look at the droppings daily. after a week i change out the feed sacks and throw the old one out. Since the birds roost in the same spot alot, if I see an abnormal poop I can narrwo it down to which bird is having a problem.
If you feed something different or they get into something in a free range sitiuation this to can change the color and consisitancy of thier poop.
If you would, if the next one is also abnormal, take a pic of it and post it or send it by email to me at 
[email protected]
Now, you guys don't start thinking I am crazy, not just yet.