1 - The Clients natural Brow hair colour. You have a couple of options with this and it all depends on what the client wants. You could;
a. Match the clients hair to her brows
b. Use the natural colour as a base colour and add in a darker shade to deepen
2- The clients hair roots- We go from the roots as that is the clients natural hair colour and what is most likely going to suit the client best. You can then also look at what other tones the client has in their hair to help decide which way you will decide to either deepen or lighten.
3- Clients Goals- With what we can see from our observations of 1 and 2 the client may have different plans. Some clients might want to go even darker or a whole lot lighter.