What you have there is old Drywall\sheetrock that has been up long enough to absorb enough moisture from the air to change the mineral content of it and change its color as well. How do I know. My garage has it too. My home was originally built in 1978, I bought it in 2000 and it was already brown out there. I have spent the last two summers painting it one wall at a time. Almost done now other than the ceiling, to cover this stuff you will need to use a very good stain blocking primer, oil base. I used Sherwin Williams Pro-Block. We started with a latex primer\stain blocker and found it didn't get good adhesion to the drywall. Basically because of how your drywall has been for the last # of years it is very very dry and is much like a sponge, if you use a latex based primer it ***** the moisture out of the primer so fast it doesn't dry right, the oil base dries much slower and I found adhered a ton better. The other problem I had and I hope you don't for your sake, was that the taped seams which had been mudded were very very difficult to paint as the old mud flaked right off as the roller ran over them, basically is was so dry that it was like a powder. It was a real problem and I had to repaint several areas to finally get even results. After you get it primed be sure to let it dry thoroughly and then it should be paintable very easy. I had to do two coats of semi-gloss over the primer to completly hide that brown color, I did one spot behind where a cabinet was going to be with just paint as I was out of primer at that point. It ook six coats of semi to cover the brown!
Good Luck and email me if you have any ?s I'll be happy to help, I know I wish that I could have had someone to ask questions of, the whole process was a real pain.
Jon