I have a Dell Xps 8700 and I was wondering if I can change mostly all of the wires into black wires. Can I do that? If I can't, which only wires can I do if it doesn't work for all of the wires.
If you have to ask that then no, You need a fully modular PSU and order custom wires for it unless you can find PSU with all black wires, many new ones have only one color, usually black ones.
I think the easiest way is to remove the PSU, tape off the PSU enclosure and plugs on the ends, and spray paint them black.
I think flat black will adhere better than gloss.