Hello,
I would also use a signal. For example create a Boolean signal in the Green or Yellow waiting point. At the begining of the simulation it will be False. So you can have a WaitSignal for this signal to be False in the Green Waiting point, then send a product and set the Signal on True. In the Yellow Waiting point have a WaitSignal for this signal to be True, then send the product and set the signal on False again.
For them to wait until there is place on the conveyor, you could use a Counter or signals again, or the component Capacity Control Entry+Exit. There are some examples of that in this discussion: How to Implement “Wait until either M1 or M2 is Free” in Process Modelling (No OR condition available)?