Cheaper alternative to highlights?

[Jemster]

My natural hair colour is mousey dark brown with a few greys coming through. I love having highlights done especially in the summer but in the winter find they can wash me out a bit. Also I just can't afford them regularly.

I'm thinking trying an all over colour but could I do this over the top of highlights? Is there a product I could do at home that would be enough to even out the highlights and roots?