I see this question mostly as it comes down time requirements. You can stick to only 1 language, or learn 5 different ones. If you can speak 5 languages fluently, that is objectively better than only speaking 1 fluently, but doing that requires a lot of effort, so most people probably won't be able to do it. In that sense, a Mage that has mastered all 5 colors of mana is objectively better than one who has mastered only 1, but people's skill usually fall on a spectrum, and learning other colors is an opportunity cost you could have spent on being better at what you know.

I think discussing whether a mono-color mage is more or less powerful than a five-color mage is silly in a game and setting as complex as Magic: the Gathering. It will depend on the person, the situation and the needs of the story.

There are always tendencies, though.

Maybe the power of a Mage isn't depending on colors, put equal to talent*knowledge*practice ?

I take the bending argument: every art is equally powerful.

A resourceful mage can block a mage's attack, but in turn a mage can either overpower or quick-think their way out of the constraints. A mage could be either more versatile or more vulnerable than either.

Keep in mind that mechanic restrictions like being unable to deal with enchantments aren't always in play in actual storytelling.