I've had my 135L for 4 years, love it. Probably never sell it. However, color rendition isn't necessarily its strongest asset. It's really just part of the whole package that makes it such a great lens. The color balance itself is a tad cooler than I like, and sometimes too green, so I compensate with some magenta bias in-camera. In my opinion, bokeh smoothness is its strongest asset. Sharpness, and both global and micro-contrast, are all a close second. I will find it hard to believe any current or future lenses in the 135mm focal length, will be a better overall lens. An f/1.4 or faster zoom of some kind, could possibly be a better overall lens, but would be pricey and hard to design and build. I find that much of the wildlife I like to shoot, needs such fast aperture for the low light...that none exist.
I certainly would be interested in a replacement with IS (which would be huge for my event work), and I wouldn't mind if they could tighten up the CA a bit. It isn't as bad as my 85mm f/1.8, but still shows it on occasion. It's interesting what you say about color. It probably is a bit cool, which is ironic, since I'm partial to my Tamron 24-70VC which is probably on the warm side.
Canon color rendition overall, is generally considered to be "cool". I've read it many places...but certainly not all Canon lenses have a cool temperature. Interesting you mention CA. My copy really doesn't have any, or if it does, I haven't found it. One troubling thing I'm noticing with the 6D, is that with the 135...sometimes AF is slower than my 70-200 f/4...especially in servo mode. Hard to believe. Also seems to hunt a bit more in very low light, than the "slower aperture" f/4 zoom. Very odd.
As far as color goes, I have never seen a color rendition and spectrum, as good as the 200 f/2L that I rented. I want one...but again, would prefer something a bit shorter that is an even faster zoom, with identical optical performance. I think it could be done, but if Canon did it, it would cost $15k !!
The Zeiss 100mm f/2 I rented, might have the best color for fall foliage, or possibly even green foliage. However, it also had some weird contrast, where it didn't go "black" enough...yet the bright end seemed very contrasty. The red end of the color spectrum was really alive and "analogue" looking.
I could really tell a difference when I was doing a head to head between the 24-105L and the Tamron 24-70 VC for my lens review. The color temperature was noticeably cooler on the 24-105L, and when I was comparing them side by side, my preference was always the Tamron (that is certainly just a matter of taste). What the 135L does so wonderfully well is the transitions in color and focus. So smooth, and the end result is typically very pleasing.