I am glad you are keeping the journal on what you are observing. Octopuses are likely harder than other aquatics to determine life stages because of their patterning ability and unpredictable behavior. The more we write down little observations, the more likely we will have a better chance of separating perceived differences vs actual differences. Photos and videos help a lot since you can go back and observe. Sometimes I am sure something is different but go back and see the animal was showing the same behavior early in its tank life, I just never noticed it.
With other species, I have noted the greying to be slowly noticeable but progressive. Once I see it, I will notice normal coloring again but see a tendency to see the off white more and more frequently. It is almost as if it is a lot of work to show match the environment and not worth the trouble after a while. I often remark that I think this is the beginning of the end with what you are observing (less the surface time but including sleeping in an exposed corner of the tank) and will still have a month or two remaining with a much less interactive animal. Aculeatus tend to live such sort lives in the tank that I have assumed a shorter senescent period but it may be age/stage at capture more than shortened senescence.
With other species, I have noted the greying to be slowly noticeable but progressive. Once I see it, I will notice normal coloring again but see a tendency to see the off white more and more frequently. It is almost as if it is a lot of work to show match the environment and not worth the trouble after a while. I often remark that I think this is the beginning of the end with what you are observing (less the surface time but including sleeping in an exposed corner of the tank) and will still have a month or two remaining with a much less interactive animal. Aculeatus tend to live such sort lives in the tank that I have assumed a shorter senescent period but it may be age/stage at capture more than shortened senescence.