Specify colour on task diagram dynamically

[Samreay]

I think it would be neat if we would set the colour of tasks (perhaps in a state handler) so we can more easily parse large task run diagrams.

Not only would this be useful in grouping different tasks (instead of a wall of green like below):

But it would also allow us to change the colour depending on the result, akin to a warning for something that succeeded, but might have taken some retries and is something we should look into. Right now we have prefects green completed, orange retrying and red failed/crashed, so being able to have a "It worked but not perfectly" that was easy to see in the UI would be amazing.