Hair advice needed from someone who knows about colouring?

I had highlights and lowlights put into my hair a month ago. I do not like the way in which they have regrown and i have decicded that i just want to go back to my natural colour of dark blonde. My question is, is it advisable to go and have this done in a salon or is there a really good colour i can buy myself? is it possible to dye over hihglights - they contained a little bleach, or will this just mean that those bots of my hair will be lighter? i want it all the same colour, any advice would be great. thanks.