Winnable arguments are backed by reasons. You can tell these because they say why. For example, "We need to make the text a different shade because the colorblind population we're trying to address can't read it against that background." Here we have an argument that points out the need of the users and makes a case why the proposed change will meet that need better.
Unwinnable arguments are backed by "because I said so". You can tell these because they don't show why something is better, merely that it is different. For example, "I'd like the text to be navy instead of grey. It just looks better." Here nothing is better or worse, just different. We need more than different to go spend effort.
If you want to be persuasive, always be prepared to explain why. It gives you a much better chance of success.