A student throws a heavy red ball horizontally from a balcony of a tall building with an initial speed v0. At the same time, a second student drops a lighter blue ball from the same balcony. Which ball hits the ground first?

I'm not sure but I think both of the balls hit the ground at the same instant.

Because according to the laws of gravitation, all objects accelerate in the same rate under gravity. Whether a heavy object or a lightweight one, if there is no air resistance, both fall with the same speeds if their initial speeds were zero. So, the have the same final speeds before reaching the ground. Remember that acceleration due to gravity is independent of mass.