Utility functions like utils.Green() can only be overridden on the package level, therefore they aren't suitable for stubbing out in IOStreams-based tests.

In #1552, I proposed to add a ColorScheme object to IOStreams. Such an approach allows us to both stub coloring (or lack thereof) in tests, as well as define a color scheme based on user's config.

Once that lands, I propose to run with that approach and convert other usages until utils/color.go is gone. This will give us some versatility for #364