I enjoy Arduino programming, but I'm terrible at the electronic side! 
How do I switch RGB Led strip to red, regardless of the feed from the existing controller? Preferably without an Arduino. I was thinking it could be done simply with diodes and transistors, but now I'm not sure.
Any help is welcome. Please keep in mind that I'm not very experienced.
What kind of RGB Led that you are using? You can get it done without Arduino chip for sure but it might be more complexity than you think. Anyway, you should try do search for the solution at first. Good luck
It's plain analog RGB strip. I have experience controlling it with an arduino, but was hoping for something simpler.
I need to disconnect all the channels from the controller and connect red to Gnd.
Any ideas?
Do you want the controller to still be able to control the red segments, or just hack it into an "always on" red strip?
If you want it to be always on, just get some Molex adapter (like a splitter or SATA converter), hack the male Molex connector off, hack the RGB strip's connector off, and splice it to the Molex plug so you can just plug it into your power supply. Be sure to fold over and insulate the G and B wires so they don't flop about and short something.