LED grow lights are designed for indoor horticulture

Metal halide and high-pressure sodium bulbs were not invented with horticulture in mind, HPS (the bright orange bulbs seen across North America)  have a large initial lumen output, but it is predominantly in the orange and yellow spectrum (less beneficial for plant growth)

But my plants look much greener under my metal halide lighting.

This is because metal halide contains large amounts of green and blue spectrum. The green light simply bounces off of the plants and into the rods and cones in your visual sensory organs (eyeballs), making it look brighter, but is in fact just wasted energy used producing unproductive spectrum.

Additionally all of these traditional light sources produce ultraviolet in amounts that are dangerously over recommended limits for these types of plants.

Green plants grown under proper LED grow lights should look dark purple or black, because all of the light photons produced are being absorbed by the plant and used in chlorophyll  A, B  and Carotene production.