Ditto the above on using 12GA for everything.
The biggest downside to using the same color for everything is tracing wires. If you end up with harness damage down the road, telling what wire is what becomes a nightmare. Even just troubleshooting issues is much worse.
If you want to limit how many things you buy, I would go to Waytek wire and get at least 4 colors. That lets you use different colors for things going to the same area and it will make things easier.
If you have multiple colors, you can pull them all at the same time if they are going to the same place (such as taillights).
