It took me a little bit of work to figure it out, but I managed to do so, and now your portals can have things like the Aperture Science Logo, Smiles, or something that resembles a flame shape, with colors like #FF0000, #00FF00, and the classic #0000FF (moving on from bad jokes), the way (I think) they work is detailed in the "How it Works" Section
I created this a while ago, but never got around to posting it. Seeing more Portal skins on the site, which did not seem to understand how to do anything cool with them (except by accident), I thought this was a great time to upload this to demonstrate the basic mechanics.
The readme.txt was made with the project, look at that for more information, but I cannot guarantee that it reflects my own opinions (at least, not those of myself right now). I explained it much better then, though, and I also explained HOW TO GET EXTRA COLORS FOR THE PACK.
I hope you all like this, though. And I really hope that someone can make an application with a good GUI so you don't have to follow the confusing instructions (probably won't happen though).
How It (Seems To) Works
First, the noise-blur skin is scaled to a certain size, then a part of that is displayed on the portal, using black as (maybe) the left pixel of the color file or a depth effect (inclusive or) and white as the other end of the color file or the other direction.
Extra Colors, Thanks To Killer_z02
Killer_z02's pack is at http://www.fpsbanana.com/skins/34587,
use the installation instructions in the readme.txt file to install it and get more colors for the pack.