The main difference between light corn syrup and dark corn syrup is the color and flavor. Light corn syrup is, well, lighter in color and has a more neutral flavor, while dark corn syrup has a richer, more caramel-like flavor. It's like the difference between white sugar and brown sugar - they're not interchangeable, but they're related.
So, if you're making something like ice cream or cake, where the flavor of the corn syrup won't really affect the overall taste, you might be able to get away with using dark corn syrup as a substitute. But, if you're making something like candy or cookies, where the flavor of the corn syrup is more important, you might want to stick with the light corn syrup.
It's also worth noting that dark corn syrup has a thicker consistency than light corn syrup, which can affect the texture of your final product. It's like the difference between honey and maple syrup - they're both sweet, but they have different viscosities.
6 Sweet Substitutes for Corn Syrup - TheDiabetesCouncil.com