You can solve this with a calculated field:
If [Value] > 90 then "Green"
elseif [Value] < 90 then "Red"
Then add this calculated field to the colors shelf. You may have to change the calculation slightly.
Hope that helps!
did that solution work for you?
After adding that calculated field all the colors are turning blue.
I'm sorry for the confusion! I'm not sure if you can do that.
But even if it's not I think i could help...
Why don't you make your own color legend picture and add it to the dashboard as an image?
Since the use-case you described is a static legend and the values will not change, this should work. Just add color to the view as normal, hide the Tableau Default legend, and use an image for your own.
It's not exactly a "solution" as much as it is a very viable work-around.
Thanks Mathew for the idea.