My scenario
  1. I have a list of thoughts that are tasks, projects, and goals (personal and professional). 
  2. I am using the appropriate types for them: "Task", "Project", and "Goal". 
  3. I use tags to separate Personal and Professional goals. 
  4. I use the tag "Completed" to mark Tasks, Projects and Goals as I finish them. 
What I'd like

When I add the tag "Completed" to any thought, I'd like the thought to change (let's say greyed out letters and soft gray background) so it's easier for me to separate what's been completed. 

Nowadays, tags can add small icons to the thoughts on the bottom right, which is very nice, but I would find it very useful to be able to change the format of the thought as described above, too. 
Interesting idea!

However, in those instances where a thought has both a type and a tag, I'm not sure how TB would know which should be the default:
> the Thought Type colors and icon or
> the Tag colors and icon.

Taking this a step further, if there were more than one tag assigned to a typed thought, then the question of which serves as the default becomes even more complex:
> the Thought Type colors and icon
Tag #1 colors and icon or
Tag #2 colors and icon.

Can you elaborate on how you would ideally like this to work?

Will be looking forward to your feedback.

In the meantime, I currently achieve this same effect by simply changing the thought type to "archive" (which is greyed out). Don't know if this would serve your purpose, but it might be a possible work-around until the tag feature you've requested in available.

If necessary, 3 archive types could also be created:
> Archived Task
> Archived Project
> Archived Goal
As Metta was saying, this can be achieved with a Type. Set a color for the text and background of the Type, and that will apply to any Thoughts you assign it to. However, if you have already edited the color of a Thought, that will override the Type's color change.

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