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What is the difference between these? The only difference I can find is that NASM warns me if I have a label on a line by itself without a colon.

I think the issue is that NASM will accept labels without colons, but wants you to use colons. It's probably just in case they want to add a feature that makes it necessary for labels to have colons and break the improperly written code.

I rather tend to think that its warning that you may have mistyped an opcode (or macro name).