Sadly you can't change the KPI text colour based on an expression at the moment. It needs to be raised as a suggestion.
What you can change is the background colour of the KPI, so you might want to look at that.
Alternatively, you can use a button as a KPI, as you can set colours on these, but that doesn't allow for two values (do you need two different sizes of text?) or a textbox. There are loads of KPI extensions out there also, if you are able to install extensions on your Sense server.
There is another KPI object in the Qlik Visualisation Bundle, which has the ability to edit CSS, but I have found that object to be less good than the standard KPI (it does allow you to trellis a KPI though).
