I typically use the leads of the capacitors to connect the 3 power capacitor grounds together on the side close to the front of the chassis, but the other connections I use insulated wire. The few wires that crossover are probably too long to use the leads from installed parts. It’s safer to use insulated also.
As far as the wire and the colors and what goes where, that is really your choice. The Book explains to use some of the cut wires from the power transformer for certain things, I would stick to that. Otherwise, I use black for ground connections and white for other connections. I include wire in the kit mostly as a convenience so feel free to use it as you want or to use other wire.