I’ve dyed my hair every color under the sun, from platinum white to vibrant red, black to purple to blue. I’ve even dyed my hair green, yellow (on accident), and orange. Hands down there are 2 colors that are extremely hard to dye over, so be careful when dyeing your hair a particular color or else you may be stuck with it for quite some time.
If you dye your hair black, expect to not go back to any other hair color for a while, unless you bleach the color out of your hair. The more you dye over black (trying to change it to brown or a lighter color), the more black your hair will become, unless you bleach your hair yourself or go to a salon to have the color stripped. Even dyeing your hair a dark brown can easily come out black if you buy a box from the store, so it is important not to leave the color on too long or else you will get a blackish color as a result, and there is no going back from there. I suggest letting a stylist handle your color if you’re dyeing your hair very dark to avoid a too-dark, flat, unnatural color you’ll be stuck with unless you’re willing to bleach it out.
Another color that is extremely difficult to dye over is red. The reason for this is because red is actually a color that occurs during the bleaching process, so you’re not just dyeing your hair red, you’re bleaching it red. If you try to dye over red to make it brown again, you often are left with reddish or coppery highlights, even if you choose to try to dye your hair black. The only way to get rid of red in your hair is to bleach it blonde (in which case you may still end up with strawberry red color or highlights), or to have your hair stripped at the salon. While red is not as extreme to dye over as black, unless you want red highlights until your hair grows out no matter what color you dye your hair after wards, consider a stylist when dyeing your hair red or getting the red out for you so you’re not stuck with red in your hair no matter what color you dye it.
Unless you’re comfortable with odd results dyeing your hair at home, and unless you’re willing to deal with a color that doesn’t quite match the box, always go to a stylist to handle your hair coloring needs. It’s worth the cost to get great-looking color in your hair.