cheap ND filters off ebay

For my GH-1 I bought recently cheap ND filters off ebay.

Then the shoot. It was a typical Scottish day: some times it was really overcast and sometimes pretty sunny so that I had to use sometimes the ND4, ND8 or even both on top of each other. These filters I've used have a lilac kind of colour tint. In order to correct in post I shot a white sheet of paper and then put the different filters on. Below is the result.

The ND4 filter adds especially more blue to the image whereas the ND8 adds both blue and red to the image but in slightly different proportions. Pretty annoying. The other option would have been to have 4 different custon WB settings in the camera which the GH-1 won't allow. Bottomline is that I'll tape a greycard to my clapperboard! Taking every time a white balance just takes too much time.