Any advice for dying already dyed auburn hair with massive grey roots. I heard that using an ash color will diminish the brassiness.

Unless you still want lots of grey, don't use ash! Do you want to cover grey? I am guessing you do. I would keep your nice auburn hue as a base, which is great for fall color, but just get your roots touched up with a matching color. Keep your base a little darker than what you would like and it should stay grey-free for longer. I recommend you see a stylist, to do that perfect base of auburn, or you can use a demi-perm if you cannot afford it, but do a good job. Then when you have faded to that perfect shade, I would some kind of gold highlights. This mights give you two different looks only within a month. My base was just dark brown, but from cool red highlights, it kind of faded to a light-medium coolish brown color, then I simply replaced the highlights I had for dark golden blonde. I have occasional grey, but gold is the best for concealing this. When it comes to avoiding grey, nothing comes close to beating Gold.

It depends how much grey hairs you have. Let's say for an instance you have 50% grey and 50& natural. You would need to mix two shades together to get the targeted result. So you would need to tubes of tint (both different shades) It is difficult to explain. But I definitely recommend you get it done at a salon. Doing it at home can create disastrous results. Lol.