If you rotate your ND filter past the ‘Max’ mark on the filter rim, it can cause an x-pattern, so ensure you’re not over-rotating. Variable ND filters also have limited application with wide angle lenses (they can only be used to about the halfway mark of Min and Max markings because the field of view is so broad). For a wide-angle lens, we suggest using fixed ND filters or ND2-32 or ND8-128. You can also try adjusting your focal length and reducing the density (rotating back towards the ‘Min’ mark) until the colour shift disappears.