I've won a State Championship with a mono-colored deck (Academy), have two other top 4 finishes with mono-colored decks (Goblins, Rogue Black), and a top 8 Pro Tour qualifier finish with a pure-black Necropotence creation. And I finished second in 2006 States with a three-color deck (Solar Peace), losing two games to one in the final. During Grand Prix Minneapolis, mono-color White Weenie dominated the format. For a newer player, mono-color decks are actually a good thing because they eliminate the need for expensive multi-colored land and they can actually be more consistent and faster than a two-color deck.
You'll just have to trust me on this one, but multiple Pro-Tour qualifiers, Grand Prix's, Nationals, and even World Championships have been won using mono-color and 3-color or 4-color decks.