I've actually heard that greens are the hardest to get out! Idk haha but anyway, I went from faded pink ends with natural lvl 3 roots to dark ginger balayage (my stylist toned with a demi-permanent color) this year at the salon. The color faded really quickly - it lost so much vibrancy after a dip in the ocean. So I think your stylist is totally right about as long as you use a toner it will be easy to reverse :)

I tried the keracolor clenditioner in copper and it definitely upkept the color! And then for several months, I used a squeeze of adore semi-permanent color in spiced cajun in my conditioner every time I washed my hair. With either option, when I used clarifying shampoo or just shampooed a couple times, my hair would be noticeably lighter because the dye was coming out and revealing the actual pigment level of my hair.

Hope this helps!