Need advice - back to my natural shade

If you don't know any laws of haircoloring I'd suggest going to a professional to have this done as disaster could easily occur.

If you want to attempt this yourself and the desired hair color is 2 levels or more darker than your lightest hair color you are going to need to fill those lightened sections with a filler to allow the target color to grab properly to the hair strands. If you didn't fill the lightest pieces you'd have greenish colored blonde hair if your using Ash color.

Remember color doesn't lift previously colored hair. If you apply color to already colored hair the end result will only be darker. This rule typically doesn't apply to high lift series of hair color as they are lift and deposit color but your hair needs to be a specific color level to get satisfactory results or you could end up with orange brassy pieces. How dark are your lowlights? Possibly use color remover on them or they need to be lightened with bleach then toned. It's a several step process to get your hair all to one color because each color of hair needs to be treated differently to get your hair all to a more similar color.

If I were you I'd goto a pro...spend the $125 and goto a reputable salon...not a Fantastic Sam's or a chain salon where you run the risk of a color disaster as most of those stylists are inexperienced or recently graduated from school.