(I am 31 and my hair is very grey)
Demi color is great! I love it for when you want a tonal change, or when you want to refresh your ends, but it doesn't cover grey very well. If you don't mind just blending your grey, then apply a demi to your regrowth, pull the color through your ends for the last 5 minutes. If you want to cover grey completely, use the same permanent shade you used last and don't pull it through the ends.