How To Dye Hair On The Thinning Crown Without Colouring Scalp?

[spooon]

Hi All,

I have a severely thinning crown but on close inspection the density of hair is actually quite high including the typical whorl pattern. Unfortunately the hairs are weak and lacking in pigment. I want to have a go at dyeing the air. I keep surrounding hair very short. How do i avoid colouring the scalp?

Thanks
S.