Alex,
This is certainly a very interesting idea. And it seems to be producing yellows fairly consistently in areas with less than 100% capacity - something I didn't think was possible. Congratulations!

Three-digit hex numbers don't work, though; I had tried them before, as well as with your view, and the game just ignores them. So it may be a Uint32 field, but the internal array is only 256 numbers big.
I should warn you of some of the experiences I've had, though. The Congestion view will sometimes produce a wrong color, even a wildly wrong color, and this has nothing to do with the color ramp being used. For example, in a city that I just tested, I have a road running at 50% capacity. With the Maxis view, it shows as bright yellow. With your view, it shows as dark orange. Both are equally incorrect. I've seen many other anomalies like this. I simply mention this so that you can be careful during testing.
One thing you mention though:
So really, the Maxis graph has no differentiation between 0% and 118%, and the congestion coloration covers an 82% range.
This is not what I have seen - I see the Maxis graph green up to 100%, and then slowly start changing to yellow. Once again, there are anomalies, so you have to look at a lot of data points. I found my Traffic Volume View invaluable for checking this out - I think it would speed up your testing by quite a bit.
Then there's the question of where you want to go with this. How far down do you want the colors to go? Which colors do you want to use below 100%? May I suggest that you coordinate your colors with my Traffic Volume View as much as possible? Users always appreciate consistent interfaces. And the colors used with my Traffic Volume View have proved very popular - I've gotten very positive feedback on my STEX upload.
In any case, what you've done is quite impressive. I'll be looking forward to hearing more results. And let me know if there's anything I can do to help.