To test this to see where it is going wrong, I would just create another text object, and enter '= vCurrentLabel', then click on your Oxxio picture and see if it shows the value you are expecting.
Also, I think there are some extra brackets you don't need in your vKleur1 variable, I'm not sure if this is affecting it, but try this:
= if ( vCurrentLabel = 'Eneco Consumenten B.V.' , RGB(210,18,66),
if ( vCurrentLabel = 'Oxxio', RGB(0,122,156),
if ( vCurrentLabel = 'WoonEnergie', RGB(209,221,27),