It won't make much of a difference at all.
The point about using an extra light DME as a base is especially true for lighter brews, IPAs, etc.
With that said, I use dark grains for the flavor/color and do use extra light DME as a late addition. The late addition is not to try to preserve a light color by less carmelization as much as it is trying to maintain a boiling gravity of about 1.040 - 1.050 for better hop utilization.
I recently made a great tasting Guiness clone. If you're interested in my step by step instructions email me for a copy.