Could be a lot of things, but you have a nice nectar flow under way. New wax by the end bars and white wax cappings.
I'm not Lauri but the difference in the color of the capped brood is due to the age of the capped brood. The picture she posted is newly capped brood. The brood behind your dark cappings are about to emerge.Lauri
Your capped brood is so nice an light colored. I checked my hives yesterday and they all had a rather dark capped brood. What causes the difference win color of brood? I do use foundations.