When I started building my layout I bought three roles of household 12 AWG wire in the three main colors that I would be using.. Red for digital, brown for ground and yellow for 16V AC
Now that I also want to run a 12V DC line I need a fourth color but I balked at the price of a whole role of heavy gauge spool of wire. I bought a shorter length of smaller gauge wire but when I came to install it, I decided that I really did need to use a heavy gauge wire - low voltage means supply can quickly run up to a high amperage current if there is a short somewhere so I don't want my supply bus to go up in smoke.
Then I had what turns out to be a really simple idea... instead of buying more wire, I decided to simply color a good length of wire using permanent markers. A light colored wire is easy to change to a new color so I took a length of yellow and made it green by running a fat permanent marker along its length a few times.
If I were starting from scratch I would buy a single good sized roll of heavy gauge wire - say white - and simply color it red, brown, black, red, blue yellow as needed.
Here is my new role of green wire and the original yellow.