You are correct that juvenile Imperial Eagle is pale buff. Many of the feathers on this bird are juvenile feathers, giving the overall pale appearance.
As Mike points out, there is very obvious moult contrast on this bird. It is clear that some of the body feathers have been replaced by dark feathers typical of adult Imperial Eagle, and the typical adult pattern of pale nape contrasting with darker breast is apparent on some of the photos. There is also obvious contrast on the wings and tail between fresh new feathers and old worn feathers (note especially how worn the central tail feathers are).
A juvenile eagle at this time of year would show entirely fresh feathers (no more than a few months old) with no moult contrast. As Mike mentions, these feathers are gradually replaced over a number of years into adult-type plumage. Birds in their 2nd and 3rd year (as this bird) will therefore show a mixture of adult and juvenile features. The moult contrast is usually apparent in the field, often at reasonable distance - look for the difference in length between old and new wing feathers, and for any missing feathers in the wings.
I hope this clarifies the situation, which is not well explained in many field guides (these can be misleading by suggesting birds are either 'juvenile' or 'adult', without considering intermediate plumages). I suspec this is why you are confused by Mike's respaonse. These are a great set of photos which show very well how this transition from juvenile to adult occurs.
