Both PAL and color NTSC are derived from greyscale television standards and represent luminance a similar way. PAL uses a slightly different encoding scheme for

Both PAL and color NTSC are derived from greyscale television standards and represent luminance a similar way. PAL uses a slightly different encoding scheme for color information that improves on some of the deficiencies of NTSC and uses a different color carrier frequency. Since the carriers are far enough apart your PAL TV doesn't even "see" the NTSC color signal and so you just get greyscale rather than incorrectly decoded color.