There is no best alkalinity for the color of the beer. The best alkalinity is the alkalinity that brings the mash to a proper pH. In a light color beer that alkalinity will be negative or, put another way, you will have to add some acid to even to a deionized water mash. Water with finite alkalinity will require enough acid to neutralize that alkalinity and the alkalinity of the malt. Conversely, in a beer with lots of high colored/roast malt the acids in those malts can be enough to overwhelm the alkalinity of the base malt requiring that either the water have some alkalinity of its own or that alkali be added to the mash (or water prior to mashing). One can in general observe that lighter beers require acid and darker beers alkali but there is not set amount of alkali or acid for a particular beer color. That was once thought to be the case but I don't think anyone subscribes to that theory any more.
Darker caramel/crystal malts tend to have lower DI mash pH's than lighter ones and there is a correlation between malt color and that pH. Perhaps that is what you are thinking of.
