Your starting point changes the route
Natural level, previous permanent color, box dye, old highlights and chemical history can all change what is realistic in one visit. A reference photo shows the destination; your history determines the route.
Skin tone is only one variable
Eye color, natural contrast, base color and how much brightness you want around the face can matter just as much. The goal is not to force every client into a simple warm-or-cool rule, but to build a color that works in real light and real life.
Say what you will maintain
If you love a photo but do not want its return schedule, say that. A good consultation can often adjust placement, contrast or brightness to protect the feeling of the reference while creating a maintenance plan you can actually live with.