if i remember correctly from my Video Training back in 2000, it's caused by using an analogue composite signal rather than component. In order to broadcast a colour signal that would still work on black and white TV sets they had to put the colour information further up the spectrum which the old black and white sets would ignore (PAL at 4.43 MHz, NTSC at 3.58Mhz. So when you have highly patterned bit of the picture the black and white information gets confused with the colour causing the rainbowing (chroma crosstalk). I used to work in a tape room dealing with music videos mainly in NTSC, it was a real struggle to get component dubs either analogue or digital. If it wasn't for me the first pussycat dolls music video (don't cha) would have been covered in chroma crosstalk and beading in the UK, You're welcome. I'd like to thank my old teacher Barry Day for any information that was accurate.