If it's an LCD TV then sounds like the Scart cable is set to RGB mode but not all the pins are connecting correctly.
If it's a CRT TV then it could be the above problem or one of the guns isn't firing (could be the tube or the gun driver circuit.....
If red is missing you'll get a green/blue picture, if blue is missing you'll get a yellowy picture and if Green is missing you'll get the symptoms you describe....
Try the TV on the analogue channels and that'll instantly tell you if the SCART lead is the problem. Also below is a link to what a scart lead/plug is
http://en.wikipedia.org/wiki/SCART