I am after some advice on grey touch up pen or something similar

[baby11]

I saw this on QVCUK.com Mimic Root Hair Colour it is a product that covers your grey roots or grey hairs that you have. Since then I have discovered you can get products in a pen etc so you can carry it around and touch up on the go.

I don't want to cover roots just grey hairs that I have, is there a similar product I can purchase without it being too expensive?

Thanks