Many people find the dark flavors hit harder and/or irritate the lungs more. This is because they are food flavorings and have colorings in for baking - think coffee flavoring from the baking dept, it adds the flavor but also makes the cake mix darker. Coffee flavoring supplied specifically for
vaping (from DV for instance) is a light tan color, it doesn't include the colorings. Caramel is another one - dark for baking, light for
vaping.
But I'd contest your statement the real dark ones taste wrong/bad/unpleasant though, the ones I've tried (like Loranns) taste great, it's just they aren't really for inhaling and most likely shouldn't be inhaled.
They taste pretty good though
However I'm talking about DIY flavors, if you mean premix liquids then try another couple suppliers, sounds like the ones you tried are no good.