Grey to me signifies a little bit of everything. As you mix two colors that were from all the light and all the dark, though regardless it's whatever your intentions are that matter. If you want to use colors as a personal categorization of what you feel is appropriate to place it in that sounds alright to me.
For instance blue magic can signify protection, calmness, and insight so I could associate meditation, protection spells, shielding, and healing.with that colr.
Black magick many see as negativity, absence of, incompleted tasks, or blockade. I could associate the color black with curses, chakra blockage, self work, worry, and different unpleasant emotions.
Though even with your own system of things magic it comes down to your intent and will. Magic itself is simply using energy of the universe to help acquire what you desire. At least it is to me.
I hope this helps.