Right, it should be dark smoke, but unless it's obviously a really serious problem, it pays to do the cheap possibilities first. Color is pretty subjective.
O-rings and crud build-up are the potential culprits in the injectors. To clean the injectors, you have to replace the O-rings.
As DeltaP aluded to, you may have a very serious problem such as blown head gasket.
If this is going to continue, we need the details of your vehicle such as the year, etc.
Do a compression test with the engine at operating temperature.