I like this idea. It would be handy to easily choose player colors like this.
I'm not sure about the "standard" part of "//Standard third party color during campaign scenarios". Can you clarify what you mean here? I don't remember there being a third party in the official campaigns.
You bring up a good point about the scoped enums. I would prefer scoped enums. I believe scoped enums have different name decoration though, so the core enums can't be scoped without breaking the linking to functions from Outpost2.exe. Any functions that used an enum in their signature would likely fail to link. This might be worth testing though.
For non-core SDK files that don't link with Outpost2.exe functions, we might want to consider changing to scoped enums. Such a change breaks source compatibility though, so we'd need to bump the SDK version number if we do this.
I'm going to suggest a non-scoped enum to conform with existing expectations for now.