Depends on the type of chart but in tables you can set the Text Color Expression or Background Color Expression to something like RGB(255,0,0)
Any expression valid in Qlik which returns a color with RGB() or ARGB() will work.
I tried writing RGB(255,0,0) in the Text color expression but then all numbers turned red. How can I turn only the negative numbers red?
As mentioned above, using any expression which returns a color will work, in your case:
If(Sum(Sales) < 0, Red(), Black())
Assuming Sum(Sales) is your expression.
The column is biult up by a master item so your suggestion sadly doesn't work.
The app was created by a consultant and I thought that I could just make the change in the format pattern or text color expression.
You can click on the link icon on the item to remove it from the master library and see the expression. You can still use the expression to change the color, but you need to know the expression, of course.