AFAIK, the grain bill the maltsters use to make their extracts aren't available, and also differs from maltster to maltster. The difference is probably beyond just color. Flavor and fermentability will also differ. It's hard to say by how much without actually testing them side-by-side. I generally work under the assumption that darker extracts tend to have more crystal malt and be less fermentable than lighter ones, although I have never done a side-by-side comparison to confirm that this is the case.
If you have an existing recipe and need to sub one for the other, you may be able to get away subbing the gold for the light, especially if that's not the majority of the extract. If you're designing a recipe, I find that the best practice is generally to use Pilsner, Wheat and/or Extra Light DME as your base and then add specialty grains for color. That typically gives you the most control over the process.