How to debunk the first cause argument without trying too hard

[Esquilax]

If we had a painting. The whole painting was blue, and every dot of it was blue, would you say it's logical to say "yes we know every part of it is blue, and the whole of it is blue...but then it can possibly be other then blue in color."

The same logic applies if every state of the universe began to exist, and all the states of it together began to exist, it's not really logical to say "well how do you know that it didn't always exist?".

That's a fallacy of composition: the temporal framework you're seeking to apply is a condition that is only necessarily true within the universe, you can't extrapolate outward so that it's true outside of the universe. In fact, given the connection of space and time, what we know about the origins of our current expansionary universe denotes a highly different configuration of space, and thus the same is true for time. The odds are actually against the fallacy you're making anyway, even if we just ignore the flaw in the logic and treat it as a serious contention.