Agreed, try to arrange to see your colourist again if you were happy with the overall colour, there are tricks she can use such as pre-softening or using a different strength of developer if your greys are resistant to colour. If your greys are particularly resistant you may find it difficult to cover them with a box dye anyway as they are not going to be better quality than what your stylist is using. (I usually recommend home colouring in most cases, but in this one where you have a stylist you are happy with, are happy to pay for the service and liked the colour I would encourage you to give it another chance until the stylist is able to perfect the formula).
Fun fact! All grey hairs are actually white (well, lacking in colour at least!) hair that looks grey is because it is a mixture of white and darker hairs, or that the white hairs are reflecting the colour of the rest of the hair!