Last weekend, during the Star City Games Standard Open (a professional MtG-tournament), there was a monogreen deck in the semifinals. Since there are a lot of great players in these tournaments, this is not insignificant.
You don’t often see monocolored decks in major tournaments. They are just so limited in their functionality. Since every color has its weaknesses, no color is good enough to have answers for everything (well, blue, maybe). Therefore, it only makes sense to play multiple colors in a tournament. After all, if you are going to try and win in a major tournament, you will face many different kinds of decks and although luck and skill are always factors, you can’t really rely on them. Going 15 rounds with room for only one or two match losses, you need to be able to answer a lot of things.
… or you need to find something others can’t answer.
Playing monocolored decks means digging deeper into the card pool than normal. How often do you see a [scryfall]Predator Ooze[/scryfall] in a tournament deck? Its a great card, but it requires three green mana, which is just too much for most decks. On the other hand, it can win games on its own. If your opponent doesn’t happen to have one of the few cards which can deal with it. (Actually, they are plentiful, but only a handful actually get any play.)
So, a monocolored deck can win. At least if the meta is right. My best result (4-0) in my new MtG-tournament career (limited to FNMs, Prereleases and Game Days at this point) was actually with a monogreen deck. Its also largely based on Predator Ooze, but with more creatures. With [scryfall]Rancor[/scryfall] on it, many decks are just helpless. Again, not all, but many.
So, personally I just prefer monocolored decks. In some ways, this is rationalizing something I was doing already for financial reasons (the manabase is often the costliest part of the deck), but I’ve also found that I enjoy the challenge. Finding enough playables in one color in a limited pool of cards is hard. So I do it for fun. I generally can’t win the best players around or the best deck(s) in the tournament, but I do have a win percentage pretty close to 50%. Since I don’t consider myself a great player, the deck has to do some of the work. So they can’t be that bad.
I’ve also played a monowhite humans deck with [scryfall]Boros Reckoner[/scryfall]s and [scryfall]Sublime Archangel[/scryfall]s, and a monored blitz deck to pretty good results.
Of course, these examples are all very aggressive decks. Sadly, this is the way it has to be. Since control decks need a lot of answers, they need a lot of colors (at least right now, in the past there have been great monoblack and monoblue control decks), whereas aggro just needs to be fast and not vulnerable to some random card like [scryfall]Augur of Bolas[/scryfall] and to dodge the midrange decks.
So, basically, if you want to win, don’t use a monocolored deck.