Color over Dyed Hair

Hair dye
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Q: I recently dyed my hair black/dark brown and was told I can't dye over it again. I have to wait until it grows out. Is this true when I only want to dye it the same color or a shade darker again?
A: If you've used a standard hair color brand (purchased from a beauty supply and mixed with developer) and you want to use the same type of product on your hair, then you should be fine to do so. You do have to be careful with certain types of hair color (henna-based and metallic salt hair colors for example) as these can react badly with regular hair color mixes.
However, if all we're talking about is a change in color using standard beauty supply colors (not the hair color kits from your supermarket or pharmacy) you should be able to safely recolor your hair. You could even lighten the hair color if you want in this situation, although lightening previously colored hair should be done with caution to avoid unpleasant results.
You should also avoid lightening the hair too much from the color you have now or you risk damaging the hair because it has already been color treated and will be more responsive to lightening.
Now if you were told by a salon professional that you have to let the hair color "grow out" you need to understand the reason you were told this. However, I'm guessing that you weren't told this by a salon professional because he or she should have explained the reason you "can't dye over it again".
Anyway, if you only used standard hair color, and your hair is undamaged from the previous color process, then you can feel free to recolor your hair. Just be sure to use the same base color in the new color as you did in the original color to avoid any unwanted color conflict problems.
See also:
How to color hair
Hair color removers for henna and black tints
How to tell what the base color of a hair coloring product is
Damage caused by hair color
How hair coloring can damage your hair