There has been a discussion specifically about the swap, but I can't seem to find it right now.
Anyway, the jist of it is that within the fog light assembly there lies the fog light unit. The fog light unit consists of a bulb assembly and a plastic cover (yellow plastic cover on early model IS's and clear on the later ones).
You should be able to change out the plastic cover (as this is the only thing that affects the color) as I have seem people change them out on their IS's on clublexus.com. However, this would entail buying a set of fog lights from Lexus (or ebay), dismantling them, dismantling your stock T4R fog lights, swap the covering, and reassemble your stock T4R fog lights properly with silicone so water does not get into them.
I have gone back and forth about doing this mod, but it just doesn't seem worth the hassle (and money). For a little more, I would rather get a set of yellow HID's from xenondepot.com and drop them in my fogs. But that's just my 2 cents.
Hope this helps.