@NicolaWeber -
I like what you are describing. Especially if the students would see the same working and colorization in their grade view.
I do not have an answer for what you are wanting to do - other than it is something that could probably be done with a javascript. If you did have something like that, it could be incorporated into your schools instance of Canvas so that it is seamless for all instructors. I do not know how any of that is done, but I do know it is possible to take javascript code and get it incorporated into the instance of Canvas at your institution (mine did it with one of James' canvancements).
As for the grade showing as Satisfactory/Not Yet Satisfactory - is that something that can be displayed already? Or are you looking for a way to have that phrasing appear based on the actual score on an assignment?