Is your hair very healthy? I would say it's okay to dye over it if your hair is in good condition.. people do that all the time after mistakes. However, it's probably not recommended to BLEACH it or do anything harsh to it. If you are just using a color to go over it, it should be fine. Whenever I dye over my highlights with a brown dye close to my natural color, it actually makes my hair feel much healthier. Good luck! Just remember that the highlights can sometimes either make the color get soaked up faster and become a bit darker than you expect, and/or they might make the color fade faster, even if it's permanent.