The colour isn't desperately important. The issue is the absorbtion at the specific wavelength of the CD laser. It is quite possible that of random marker pens from a stationary shop, some will absorb at that wavelength, and some will not. Thus it is quite possible that you could get a green marker pen that will have no effect whatsoever (eg an ohp marker, as these are meant to be transparent (or at least translucent)). The CD Stoplight is an opaque, matte finish for those who've not seen the real stuff.
Of course, I have no doubt that they're raking in the cash and it is costing them pence to make. However, I also have no doubt that it works for me, and that the price (around 10p/CD) is very reasonable for the level of improvement that I (personally, IMHO etc) experience from it.
Going into theory now, with DVD and SACD things are the same but different. The same in that the whole thing should work (or not, depending on your pov) for the same reasons, but different in that you need to absorb at a different wavelength.