Either your white balance is pretty out or as in this case your using a cheap filter...
I've been out on a few trips with a couple of mates recently. On a trip to Lyme Regis, one of them asked why his pictures were all coming out with a heavy pink colour cast. My other mate and I had a quick look and at a glance there was no reason for it... However after looking more closely, he was using a cheap eBay ND filter on his lovely 16-35 Canon. His reaction to this was, ah I'll Photoshop the colour out/correct it. We both said fine but suggested trying one of our filters. After a bit of faffing about we found an adapter ring and he borrowed a Cokin Z style holder and a few of my grads, immediately seeing much better results.
Moral here.... Dont put cheap rubbish in front of expensive glass. Allot of things can be processed out but there is much more satisfaction getting the picture near perfect in camera
P.S, he was shooting with a 6D. It a very very nice camera! My other mate was using a 5D mkIII, very very nice too but both were envious of my flip screen and went away with aching necks and knees