My hair is a mousy light brown colour with an increasing number of grey hairs so I've always had a half head of blonde highlights put through it to slightly lift it. I currently have about 5cm root growth at the top and I honestly can't afford to keep having highlights done. The rest of my hair (shoulder length) still obviously has the highlights in. My question is can i just put a dark blonde box dye over my whole head now to save the cost of going to the hairdressers for highlights? I'm really clueless about this so would really appreciate any help! Thanks 