If you have not done so already, you can add pH to your list of things to worry about before you make stock.
The glasses in the photo show five samples of one batch of chicken stock. I adjusted each to a specific pH with either lemon juice or sodium bicarbonate (baking soda) so that pH of each rises from left to right.
The most striking thing you notice is that the color darkens as the alkalinity of the stock increases. There were also significant differences in clarity which did not come across in this low resolution photo: the more acidic samples were more cloudy.
None of this surprised me as it was all predicted by the science of the electro-chemistry of colloidal systems. But it's certainly worth running up the blog pole as a caution to anyone who is tempted to jazz up their stock with acidic ingredients like vinegar, lemon or canned tomato.
For optimum color and clarity the pH of all the ingredients in your stock should be as close to pH 7 (neutral) as possible.