The Tiffen 4x5.65" Neutral Density (2.1) Infrared Color Core Glass Filter appears grey and reduce the amount of light reaching the film. They have no effect on color balance. Color shift is a critical factor in digital photography. This color shift is directly affected by the infrared spectrum. Specifically, in high definition photography, the sensitivity of imaging sensors to IR has become a challenge. This is particularly evident when using standard neutral density filters that absorb visible light but pass infrared.
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.
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.