Reflection gel
Wait, the gel should appear bright white? I always get a purple and black checker texture on them (which I actually kinda like, it goes well with the purple splash particle).
Aperture Science: We do our science asbestos we can!
LOL
About white-bright.... hmmmm I think you are right: a missing texture shows a chekerboard by default, and a lack of one custom texture is what causes the bright-white 
(when for example you don't pakrat it into the bsp and such) Thanks for the correction! 
useful tools and stuff here on TWP 
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We should probably come to a concensus on the gel's color, with so many out there. Right now I know of maps using purple, bright white, normal gray, neon green, yellow, and black. Are there any others I'm missing?
Wait, where was the yellow and neon green used? I thought the rust and lime colored gels crashed when you walked on them, I made a map to test that.
Aperture Science: We do our science asbestos we can!
I think that the dark grey/black reflection gel used in carl.kenner's Style Changer mod is the best/most common, so maybe that's the one we should accept?
In the unreleased mod Conversion, and probably Back-Stock as well. The color was changed with portal_paint_color.
And I agree about the black/dark grey, if it's in the Stylechanger. Anyone else?
I did choose black because it's one of the main colors used in Portal 2 (Orange, Blue, White, Black). Grey could work too, but I just don't like the look of it. But silver could look quit nice. Like a liquid mirror or something (cubemaps). Anybody who wants to try that?
The problem with "portal_paint_color" is that it also changes the color of the conversion gel... And as far as I know they are no alternatives to this.
Unfortunately there aren't.
Sprowl: I haven't found a way to make it use the cubemaps, since it doesn't seem to load any special materials. But that would look really awesome.