Well yes, but they’re distilled. It’s as if you said “the two main ingredients in soap are fat and caustic soda”; in properly-made soap you will never actually find any of the caustic soda it was made from, and you might not find any fat either. Distillation of liquor destroys the original ingredients such as potatoes or whatever. There is still potentially chemical evidence of what it was made from, but never a lump of potato or a grain of barley. 
A better, more apropos example: refined sugar from beets and refined sugar from cane are exactly the same sugar. You can tell which is which only if they are impure. In the same way, potatoes, grains, and fruits all distill down to the exact same alcohol, if you’re taking care to purify the final product.
In very broad very over-generalized terms, darker drinks have more impurities intentionally kept in them. (the easy example being dark rum vs white rum)
Well, depends on what you mean by “impurities kept in them.” It’s more like added to them, if we’re being generous. Light and dark rum both start off clear. Dark rum is aged in charred oak barrels (like some whiskies), after which it acquires additional flavors and a dark color over time. (Or really cheap “dark rum” just has caramel color added to it.)
Right. I did indicate that what I said was too general.
What you’re implying (namely, that all liquor is nothing but vodka with flavouring added) isn’t true. Scotch whisky is different from bourbon, even straight from the still with no aging, and the reason is impurities.
If that’s how you read it, my apologies for not being clear. My post only applies to your post about light vs dark rum, and that darker drinks have more impurities intentionally left in them (most non-vodka clear alcohols do as well.) Of course liquors off the still still have various flavorings in them from fusel alcohols and non-ethanol components. You absolutely can tell the taste of the original fermentation source unless you purposely distill it to remove all impurities. But it’s not a light vs dark thing.
nm
Re-reading your post, perhaps I’m making too much of an assumption as to what you meant. I read it as the difference between light rum and dark rum being that dark rum has more impurities left in it (perhaps giving the implication that the dark color is caused by the impurities.) Dark colored alcohols do tend to start with a more flavored clear distillate (I mean, I haven’t heard of a dark barrel aged vodka, for instance–though I’m almost sure if I Google it now, I’ll find something), so, in a way, starting with more impurities (not in the case of rum or tequila, though, as far as I know. They all start the same but are not aged or aged differently.) But I want to make it clear that color is not a good indication of the “purity” of the ethanol. Plenty of drinks (like gin) are distilled and clear and have lots of non-ethanol flavors to them.
ETA: Yep, sure enough, barrel-aged vodka. Apparently popular since the 15th Century in Eastern Europe, though apparently not popular in my Polish family.