MH (metal halide) and HPS (high-pressure sodium) are two types of HID lights that have different spectrums and effects on cannabis plants. MH lights produce a cool white light that is rich in blue wavelengths, which stimulates vegetative growth and branching. HPS produces a warm yellow light that is rich in red wavelengths, which stimulates flowering and fruiting. For vegetative growth, MH lights are generally preferred over HPS, as they promote more compact and bushy plants with shorter internodes. However, some growers use HPS for vegetative growth as well, as they can produce faster growth and larger leaves.