depends on the amount of ND on the camera. Once you go past .6 ND you can start to get IR haze and most things by .9ND will show IR issues in the digital world. The funny reds are from the Ir contamination.
"...Unlike many photographic films, the CCD or CMOS of a digital camera is inherently susceptible to infrared contamination, even when the manufacturer attempts to reduce this problem by adding an infrared absorbing filter on top of the image sensor. Because these filters do not have a sharp cutoff at the infrared wavelengths there has to be a compromise in order not to exclude visible red wavelengths. This results is some infrared wavelengths being allowed to pass.
While minor infrared contamination does not normally cause much of a problem for visible images, when using conventional neutral density filters especially denser grades, this issue is multiplied by the filter factor."
