equipment recommendation - How do I chose a vari-ND fader filter without color cast?

I would like to buy a vari-ND fader. I have maximum $70 for it so i can't buy the high end faders.

I try to get every information about:

• Polariod 67MM Neutral Density Variable Fader


• Nature 67mm Fader-ND


• Opteka 67mm HD Variable

I found this site, and see Polariod the most color-casted. http://cheesycam.com/nd-filter-color-cast-testing/

Any suggestion? How to choose? Which one may I pick? Or just buy some "alone" ND filter? (0.6; 0.9;)