Changing Gel Colours
Well the way that Omnicoder and Reepblue have made gels work is as I was planning to do; the entire map must be prepared as if it were a grid with sections being activated to produce your effect when paint is detected in those areas. Unfortunately paint from an info_paint_sprayer according to Omni can't be detected so you have to use prop_paint_bomb as a template to reproduce itself. Yes, this is very inefficient, easily breakable and not a complete solution but until Valve give us access to modify the code it's the only way I see possible.
On another note, the particle effect is ready. Unfortunately I am having trouble overriding the standard VPK one. Omni told me that bspzip/packing the replacement file overrides any VPK version but that doesn't appear to be happening for me after using multiple packers with 0 errors. Inching closer and closer to VPK editing. 
Another update.
I gave up trying to override the VPK and edited it. The custom coloured box is now complete as is the splash effect. Unfortunately the splash itself is STILL the wrong colour so it seems my almost certainty about the third unused colour in the effects file being used for the final splat colour was wrong. Still got to find it before I can edit it. Any ideas?
Are there any tuts for like programming things like gels, or somewhat specific? The one on the Valve wiki is mainly only about binding keys, not too helpful...
Sorry, where are you finding values to change the splash/world paint color? I'm doing something completely different but that also recolors the stick gel and would really like to know. I found the blob colors, easy enough, but what particle/script do I look for for the splash/paint?
Thanks.
Update:
I have now been through every single material & texture, particle effect and script. I cannot for the life of me figure out where the final splash on the ground effects colour is determined. I say 'every' to mean, 'all common sense places for where it would be,' but it looks as if either Valve are monkeys banging on a type writer and the colour file is in some completely innapropriately named place or it is being determined in the program itself. If the first is true, I am too tired of this chase to continue searching. If the latter, well as I keep repeating we'll have to prompt Vave for modding ability. Either way I now have enough to put together a heavily modified demonstration of my idea. Hopefully if enough people do this it will make Valve consider giving us the access we need to make our ideas a reality.
Additional note, I have also not managed to find the effect for when paint splashes on a player and the colour appears on their screen.
If someone knows of a command that provides a printout, or overlays the gameworld with a grid like navmeshing that tells you the name of the effect/material/script/w.e in action at that point in time it might help. Unfortunately, I know of no such script and until then cannot figure out the paint splashes source.
Last note for those who find this thread and wish to continue the search, I do NOT believe the splash is 1 simple material. As stated, you can have small sprays of a paint when you use a paintbomb and only when enough of an area is covered does the effect come to work. As a result I'm almost certain it is part of a script.
I will continue to check this thread for updates and I will be posting all I discovered in the next few days.
@TheDarkness73 - http://www.leeland.net/l4d2-scripts-pla ... c-toe.html is the best available example.
@MrHappy - I have only managed to modify the colour in the particle effects, the blob and the cube. I need to take a break from searching for how to modify that splash because as far as I'm concerned it is now as stated determined in the program itself somewhere.
Yes I was afraid of that, there are definitely shader maps for the surface paint but not color. Probably hard coded in the dll :*(
Does this mean I can't retexture the portal gun?
Does it look like this?
http://forums.thinking.withportals.com/viewthread-t3487.html
If yes, then I guess some old files where left in.
@MrHappy - My worry too sir.
@YoHoat - Yes, you can and people already have. It requires VPK editing at this time though.
@AnotherBadPun - I can get that easily. I'm going for changing the entire colour, not simply adding the purple blob colour.
I've taken the last resort step and am now asking Valve. Hopefully they'll give a solution or tell me it's not possible so I can completely give up the search. (Unless some genius comes along with a way to override the paint colours.)
I can see that Electric Gel and the idea of Sticky Gel being mixed, the gel is lethal until you magnetize yourself when you can then walk on it for maybe a 30 second time limit at which point you fall to your death/get electrified. Think Iron Boots from Zelda Twilight Princess with the walking on walls/ceiling.