At first the kernels of the berries are green and the outer covers are a paler green, and cover less than half the kernel. To me, they look more like miniature acorns at this stage.
As the berry ripens, the kernel swells. The covering layer begins to redden and to advance down the kernel.
Riper = yet redder cover with yet more complete coverage.
Varying degrees of ripeness in berries which are almost touching. Perhaps it was the time of day I was there, but there seemed to be a direct effect that the berries in the sunniest spots had come furthest along.
These pictures all taken within a few minutes, of berries on the same yew tree in West Park.
Hat tip: Jefny.