Ok, problem resolved!
The issue is the following:
While the qlik documentation says this should work:
if(Sum([Sales]) < 10000, 'red', green())
Even the color functions need to be written between single quotes.
So this is how DOES work:
if(Sum([Sales]) < 10000, 'red', 'green()')
I just spent 4 hours looking for this... Probably this needs to be rectified in the documentation or in the programming of the tool.