So, after reading the responses from my original post. I went down to the ATT store where I got my replacement Lumia 900 updated version for a low volume mic and they told me that the screen was normal after three reps inspected it and compared it to other Lumia 900's in the store that were not purple. They said that it could be because of the software update. I told them that my original phone did not turn purple in low lighting and others that have updated the software did not turn purple as well, but the still did not approve the replacement.
So, I went to another ATT store and told them about my purple screen and did not mention anything about the software update and they compared it to other lumia 900's in the store and approve my exchange. So, dont mention anything about software. There is definately something wrong with the screen. My new 900 does not have a purple screen.