So one way to do this is to set up 2 calculations for the text...
In the attached (T10) example, I've created 2 calculated fields
[Sales <= 10000]
[Sales > 10000]
I then place them both on the text label tile, and edit them as per the below image
As one will always be NULL and Tableau doesn't/can't plot NULLs we get the effect we want.
Hope that helps and makes sense.
Conditional Formatting of Text.twbx 467.8 KB