You don't need nearly as much light during vegetation as you do during flowering, so having less lumens with the MH light will not be a problem. Besides, lumens aren't an accurate measure of how much energy the plant can derive from the light, especially if the color is different.

The spectrum number is given as a temperature, ie, 7200K or 2100K. This is the temperature in Kelvin that an incandescent filament would have to be in order to give off that spectrum of light. You don't really need to know that though, just remember that a higher number means a bluer light, and a lower number means a yellower light. A number like 4500K would be more neutral. Sodium lamps and metal halide lamps release a bunch of discrete wavelengths of light, which makes them not so good at rendering colors, but great for things where efficiency is paramount, like for grow lights and street lights.

But the main thing it comes down to is what it'll do for your plants. Blue light stimulates chlorophyll production more, making your plants better at absorbing light. Your plants think they are seeing the blue sky and so they grow bushier to take advantage of all the direct sunlight they think they're getting. If the light is orange, on the other hand, the plants think they're getting mostly reflected light, so they stretch up, hoping they will eventually be able to see the sky. You want your plants to store up their energy for flowering, not to keep stretching toward some sky that isn't there. Once you start flowering though, the orange light will lengthen your plant's stems so it can fit more buds on there.