When painting blonde hair, you will want to use similar colours to what you have used on the flesh of the face to provide continuity and prevent the hair from looking fake. Evaluating the colours you have used in the background of the painting will help you to shade appropriately and bring out the right colours in the hair; for example, a blue background may make blonde hair appear almost grey and a grey raw umber mix may be right. Use colours from the background sparingly when painting the hair. Make sure the edges of the hair are not so smooth as to give a cardboard cut-out impression.
Reserve the outline of the hair with masking fluid when drawing a pencil outline of the painting. Generally, watercolour artists will then paint the face first, leaving the hair until afterward.
Wash the hair with a colour wash of Lemon Yellow and Cadmium Yellow Pale, with small amounts of Raw Sienna to suggest shading. Leave it to dry.
Paint the hair with the same colours used in the face, namely Burnt Sienna and Alizaron Crimson, using quick light brushstrokes to suggest darker strands of hair showing through. Blend them well so they do not look harsh.
Add white and a touch of Yellow Ochre for highlighting lighter areas.
Use a shade of Windsor violet to add shading underneath the hair where the shadows touch the skin.
Scratch a few lines down the hair, to make it look really textured, using a sharp knife.
Using a putty eraser instead of a conventional eraser will help protect the watercolour paper.