Why do we use purple and pink on the advent wreath?

Why purple and pink, why not green and red?
Why is santa red, why not green?
Why are red and green the traditional colours?

Good questions...

Well, red and green I found an explanation easily:
(From another website)

"Red symbolizes the blood that Jesus spilt to redeem us from our sins.

Green is the color of life. Therefore, green symbolizes the potential for eternal life that Jesus' sacrifice made possible for all of us."

As for purple and pink, I can find this information.

Traditionally the candles are purple, because in antiquity, purple dye was very expensive and it was the color of royalty. We use purple for Advent because it is the season of the coming of the King.

The pink candle is becoming more and more popular, but it has a strange origin. Long ago, the pope had the custom of giving someone a rose on the fourth Sunday in Lent. This led the Roman Catholic clergy to wear rose-colored vestments on that Sunday. The effect was to give some relief the solemnity of Lent, so this was a very popular custom.

Hope this helps