There's probably at least 10 different ND Filters around the studio, most of them being a type of 'Variable ND' or 'Fader Filter'. They are just so handy at getting the right exposure. Some of these filters use glass that will cast a purplish color, some more than others. Besides being able to confirm through reviewing your videos, here's another way we found. It's hard to see much difference by just holding it up to the light, and for some reason using the reflective surface from a Macbook works pretty well. As you can see two of the four don't have any color cast even when we played around and dialed them to different ND stops. The other two always have this color cast even when dialing them up or down.
Keep in mind that this does not solely apply to 'Variable' filters, but a color cast can appear in static ND filters too. For static ND filters, it would be very obvious and won't require you to flip them around. So I know you're wondering, which two had the color cast? Polaroid ND Filters and Nicna (sometimes called Rainbow Imaging). The Polaroid was actually the one with the most Purple color cast. Which ones did not have a dramatic color cast? Fader ND and Nature did not cast. Of course we all know you can't go wrong with the LightCraft Fader ND Filters, but for a slightly cheaper price I'm quite fond of the Nature brand Variable ND filters which you can find below.