First time using GND for landscape... Why did it turn my clouds purple?

I bought some Cokin GND filters and used an 8 for a picture of a landscape in the morning. It worked great... Except that it gave the top of my clouds a hideous purplish tint. :concern: It only did it towards the top of the picture (darkest part of the filter). I don't know why it did this.. Any suggestions on how to prevent this? :/

Images are needed but it is probably a white balance thing...

1st is without filter, 2nd is with. They have the exact same white balance setting. Maybe I just have the tweak the white balance a lot when I use the filter? I messed around with it a bit and was able to improve it, but there was still a purplish tinge towards the top.