If there is actual sputum in the culture, and not just saliva, it really doesn't matter what it looks like. After six months of medication, my cultures were completely transparent in color, but still positive for MAC.
I don't know what the culture results would be now, after 4 years of saline. My doc says as long as my sputum stays clear and I am completely asymptomatic we will not do another culture. I'm not sure other docs would agree, but she has been helpful to me in getting my asthma under control (that was a 20+ year struggle) and doesn't hesitate to return my calls and discuss any concerns.
Sue