Let's see if we can't organize this a little bit, and in the process get down to first principles.
Yellow chicken stock won't make risotto turn brown, no matter how intensely flavored or how yellow. On the other hand, if the stock itself is brown it can't help but turn the risotto brown as well. It doesn't get more basic than that.
Neither the size of your pot nor the amount of chicken you stuff into it as anything to do with the color of the stock -- and is only indirectly related to the intensity of the flavor. To make a good, strong stock you do need a sufficient quantity of chicken parts -- but it's mostly a matter of proper skimming, low enough heat, and enough time. Try and remember that a good stock is brewed more than it is cooked. If it ever boils with the chicken in the pot, you messed up.
Improper cooking technique -- for instance cooking too hot and leaving the scum in too long without skimming -- can make a chicken stock cook tan instead of yellow. Boiling will mix little, itty bitty particles into the stock so thoroughly they can not be sieved out.
If you want a true, "white" chicken stock, i.e., blanc, make the stock in the usual way using celery and raw onions but without carrots. You may, if you like, substitute parsnips. Avoid bouquets garni and herbs generally. But a handful of parsley thrown end near the end of cooking can be a good thing. It has some weird magic which partially cancels the pale yellow from the chicken and makes the blanc look whiter.
Blancs aside, assuming you are not messing up your stock or using a brown stock (a brun) to begin with, the undesirable color most likely comes from browned (caramelized) onions and from the fond they left in the pan.
If you're using butter to cook the onions, or to coat the rice, the color may also come from the butter itself browning. Try substituting olive oil.
In any case, keep the heat down. As FF said, sweat the onions rather than browning them. Try cooking the entire dish at medium (or less) until you have complete control of the process.
Hope this helps,
BDL