Can electrical cables in wall cause the wallpaint to change color?
Most probably from temperature. Electromagnetic fields usually do not change plaster or color in that way.

If your installation is old and plastered in, and undersized, the wires would beat up and the wall material would e.g. loose some Cristal water over time and thus change color.

If you can verify your hypothesis with some metal or wire detector. If you think it is best, try to find the consumers for the wires and turn them on. After some time you should be able to notice a difference. The ideal way would be a thermal camera, but they are quite expensive and you probably cannot easily get one.

If it is temperature induced, you should consider replacing them with an up to date Installation and make sure you are not overloading them.