I've tested the Hitech filters in varying conditions now, and can safely say if your shooting with the sun behind the camera there isn't really a colour cast. However the second the sun hits the filter from the side or front, the colour cast is awful!
LEE Filter System, Hitech 10 Pro Stopper ND, Hitech 1.2 Soft Grad ND
A selection of my work can be purchased from duncangreen.co.uk. Prints are limited edition.