Provide additional option for selecting color on grouped view

Things seem to be working well - thank you!! One thing that I think needs to happen from a UX perspective is to provide a message if the user selects an option but the underlying data doesn't support it. For example, if there are no fields of a select type, then we should have a message that no eligible fields were found. For example, all three options show up for me for a view based on Views CSV Source.

Added a warning message as the default placeholder for colors. We might need to tweak the wording though.

Thank you! There's a minor issue where if you move from the third option to the second (on a taxonomy term field), then it looks different than if you move from the first to the second. But I'm going to make it a follow-up issue.