If you have years of highlights in your hair ( and it's naturally fairly blonde anyway) but want to go dark brown just to see what it would look like, would the best thing to do be to use one of those dyes that last 6-8 washes? I want to go dark for a change but fear it won't suit me. If I don't like it, will I just be able to go back to the hairdresser and get highlights put in again and it will be back to normal?
In my experience the semi permanent dyes are no good for showing you the real colour your hair would be if you later went for the permanent, especially on pre bleached hair.
I would ask the hairdresser their opinion ~ they know what works and what doesn't. How about trying out a wig or maybe a hairdresser with computer imaging to see how dark hair looks on you?
If you do decide to dye your hair and then discover you don't like it, I imagine that blonde streaks would be the answer ~ not highlights, but the chunky streaks. Again, I would recommend getting the hairdressers advice first :o)