AskHistorians

It's a result of our voting system, first past the post takes all. The party with 51% of the vote gets the seat (all of the representation), parties with 49% of the vote or less get nothing at all. This is a "winner take all" system. When you get nothing when you lose, being on the side of the winning candidate is really important, so people don't care to align themselves with anyone other than the top 2 candidates. This mostly comes up when a candidate tries to "split the vote". A candidate splits the vote when they are popular within their party but didn't win the primary, so they try to contest their party's candidate by running in a third party. Splitting the vote is considered extremely poor strategy, because what happens is you get a 50/25/25 split. As in, almost nobody in the other party will vote for you, so you are competing with the other candidate for 50% or less of the voting population. It's a losing proposition by default.

Edit: oh if you mean the literal colors, yeah those are our flag colors. politicians wear them to indicate patriotism.