graphic problems!

I really want to know something ...

I bought NightFall by the Guild Wars store, the one when you open the Guild Wars Window when you start playing.

Before I bought this game I was playing Propheties and my computer never got visual problems ... Now when I go in Kamadan, walls become green for 1 to 2 seconds, outside towns, ground become green or blue or orange few seconds, but it's not alway, just sometime ...

Is it my graphic card, something in my computer who is broken ? I play also Battlefield 2 and other "big" games for a video card and everything is Okay, it just when I play Guild Wars NightFall ...

Someone can help me ?

You know, it's really not possible to help you if all you tell us is that you have an issue with the colouring, and that you bought nightfall from the online store. You have to give us info about your pc, like the graph card, memory, os and stuff like that.