Generally its kind of tough because a lot of the extracts don't give you proportions, but some at least tell you the grains they use and you can figure it out from there. I know Briess is pretty good about listing the grains that are used in the batch, and from there you can try to figure out the proportions based on the gravity and color of the beer. If its just extra pale or pale extract, I'd just substitute your base grain of choice. Depending on the style, I'd feel free to sub based on the region (pils for a German/pils beer, MO for a british beer, etc.). Some extracts also contain some carapils, but I'd leave it up to your setup and if you thought you might need a little head retention, either some carapils or a small amount of wheat should help.
Of the amber extracts I've tried, most mention having at least a mid-crystal (40-60), and I think some even mention using some darker mashable grain (munich, vienna, etc.). These are gonna be tougher, but you could always add enough base grain to get to the OG in the recipe, then add your crystal until you hit the right color. Same would go with dark extract except its gonna contain some higher crystals (60-120) and some black patent or other similarly dark malt.
