dark brown and purple top leaves and stunted bud growth

[zebu111]

There's a good chance that this bud was subjected to nutrient solution burn. These symptoms are seen when the EC concentration of hydroponic solutions is too high. These symptoms also appear when strong nutrient solution is splashed onto the leaves under hot HID lamps, causing the leaves to burn under the solution.

Marijuana Plants Problems - Yellow Leaves, brown spots, nutrient shortage etc.