The colors on the contribution graph are determined by looking at all of the days currently being displayed, and then allocating shades of green relative to what’s there.
That means that if you have a new day with a record amount of activity, much more than your previous days, that day will become the darkest shade of green, and all of the other days will become lighter.
Similarly, if your busiest day drops off the end of the graph, a new busiest day will be selected, and some squares will become darker.
The cut offs for the different shades of green are calculated separately for each user based on his or her contributions over the last 365 days. We use quartiles after removing outliers. This means that one user might get the darkest shade of green by making 5 commits, while another user will need 50 commits to get the same shade.
I do apologise if I am repeating myself, but I’m afraid we do not discuss the details behind our algorithms as that would allow certain users to game them. I’m afraid we won’t be able to share more than that.