We booked in August for the following June. July and August are busier months so you might want to book a full year ahead in Yellowstone. Colter can usually be booked about 4 months ahead. Your plan sounds great to us! We've done Colter and the Lake Yellowstone Cabins. In June, we'll redo Lake Yellowstone Cabins (obviously, we liked them a lot!) and do Mammoth cabins. In 2007, we did Colter. Our experience with the CS resturant there wasn't good, but we loved getting food at the little general store.
One tip at Colter Bay-be sure you spend some relaxing time sitting on benches at the Marina admiring the scenery-
We loved visiting Yellowstone and the Grand Tetons. In 2007, we'd planned to visit a different National Park every year. We got as far as visiting Yellowstone and GT and haven't been able to tear ourselves away to visit anywhere else yet.