KPI objects allow you to set colours and formats in the properties.
Select Custom format and type:
Then go to Appearance, turn Conditional Colours On and add a limit, type 0, click on the left colour and select Red, then click on the right colour and select Green. It should be something like this:
Your KPIs should look like this for example:
Instead of Selecting Custom formatting and specifying the format I've shown before you could've also set the format to Auto and have num with something like this:
KPI Objects don't have the Background and Foreground colour expressions that other objects have, a table for example, in which you could've done this:
In which I've set the colour in the background to something like:
I hope his helps,