First choice would be to buy a used bike harness.
Personally, I don’t use ‘automotive’ wire, especially the crap sold a your local auto store. If you do use auto wire, go to the nearest ‘pick and pull’ yard and buy a chunk of a harness out of a junk car with the colors you need. OEM wiring is much better the auto store stuff. You might even get some nice connectors too. A lot of the OEM wiring today uses waterproof connectors, similar to Deutsch connectors. Good stuff.
When I’m doing a bike harness, I use marine grade wire. Much different, relative gauge sizes are a little heavier, and the wire is tinned. Marine wiring generally uses only crimp terminals, not soldered terminals. Better. Soldering makes the copper brittle at the solder, and vibration can fracture the wire. (I also do marine wiring to ABYC standards.) You can buy lots of different colors of marine grade wiring in small reels online. I buy a lot of marine wiring stuff from Defender Marine.