Posting for a friend: she made cupcakes and had just enough dark paper liners but one so she put in a pink one to finish out the dozen. She one cupcake in the pink paper came out with a nice domed top while the others in the dark paper were flat. She did say the darker liners were a little thicker paper than the pick one was, but would that make that much difference?

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It must have...If they're thicker they'll probably affect how the heat hits the batter and will slow down the baking, if that makes sense.

Is there also a size difference?  The darker one looks slightly taller and perhaps a smidge wider.

Nancylou I noticed the same thing. I have liners that are taller and they need way more batter to give a nice dome. 

I ran into that exact situation last weekend.  A pack of identical liners in the same pack but different colors.  The dark colored ones burned on the bottom because they cooked faster.  I assume it is the same principle as baking in dark colored tins as opposed to regular tins.