Return `description` of instance not whole ColorArgument Enum.
Return `description` of instance not whole ColorArgument Enum.

Commandant’s help headers take a format like:
```
[ --key defaultValue.description ]
	usage
```

Move description of all valid values for `--color` flag into Option usage.

NachoSoto/Carthage,erichoracek/Carthage,abbeycode/Carthage,abbeycode/Carthage,NachoSoto/Carthage,erichoracek/Carthage,erichoracek/Carthage,flovilmart/Carthage,abbeycode/Carthage,NachoSoto/Carthage,flovilmart/Carthage,flovilmart/Carthage,erichoracek/Carthage,abbeycode/Carthage