If you want very simple option use the "Visual Cues" tab.
For more complicated scenarios If you click the + sign to the left of your expression QlikView will give you a drop down of options. Click on "Text Format" and you can then add expression you want:
=If (FieldValue > 10 and FieldValue<17, RED(), IF ( FieldValue >= 17 and FieldValue <=100, RGB(128,28,105)))
Notice there are some colors that you can just call like RED() or you can use RGB function and utilize any color you want.