Any idea about substitution between dark and light corn syrup?

I think you'd get a more brown-sugary flavor with the dark. A candy thermometer will help take out some of the guesswork that might be involved if you were judging doneness by color alone. I like to make peanut or almond brittle my making a simple syrup out of butter, panela, a dash of salt, water, and dried ancho chilis. Panela is one step more brown-sugary than brown sugar since it's just pure cane juice cooked down until it's solid. You can find it in latino grocery stores and some natural foods stores. It's hard so you have to shave it down with a sharp knife or a good grater. Adding some to your corn syrup might give the brittle a more golden brown earthy flavor.

The only difference is the amount of molasses remaining in the syrup. Switching to dark will give you darker peanut brittle with a richer taste.