although I can't answer your main question, I'm not sure if you really want to adapt a cold light source to a Multigrade 500 head. A cold light source is one that does not have a conventional bulb (and thus produces far less heat - the reason why it is called a cold light source). It is usually a discharge lamp, similar to a neon lamp. And I know of no colour head that uses a cold light source.
People usually use cold light sources as an alternative to a condensor on the one side and a diffuse color head on the other side. A cold light source does have the same diffuse soft light as a color head but is simpler to handle and does not require active ventilation. And a cold light source is usually much more efficient (it is brighter and does emit more blue light, to which paper is most senstive). The drawbacks are a reduction of dodging and burning times and/or lower and worse perfoming apertures on the lens.
You also have to take into account, that the different spectral composition of a cold light source changes paper speeds for Multigrade as well. Grades 4 and 5 will take double exposure time with normal light, because the amount of blue light needed to expose the "hard" emulsion part on the paper is low with a normal bulb. This is no longer true for a cold light source. It is not a serious problem, but it does require some adjustments and you'll have to become aquainted with it again.
BTW: the usual cold light setup for a Laborator 138 is the use of LACOLI/LACORTA, but this will throw out your Multigrade head.