I agree with the others... If it was bright yellow, it was probably a lot of stomach bile and since it was mixed with grass he probably had a tummy upset... Dogs (and Cats) will often eat grass to settle their tummy. (Not always a good idea depending on the amount of chemicals in the grass!
)
If Murph is still having problems and you need to take him to a Vet, DM me and I will give you the number of the one I use here in town. They are very compassionate and understanding.
*HUGS* to you and your best buddy.