What happens if I leave my hair color on longer than specified in my instructions?

It's important to read your instructions carefully before applying color so you know the details of each step. However, leaving color on for longer than instructed can have negative results.

If you're using Demi-Permanent color, your hair will typically become darker as the color sits on it longer. If you're using Permanent color, the color will also become darker. However, leaving Permanent color on for too long could damage your hair and affect the way it absorbs color.

Questions Related to: After Coloring
I just colored my hair, and my color is a little darker than I wanted
What happens if I wash out the color before the time limit specified in my instructions?
Is it normal to see some residual color in the shampoo 24-48 hours later?
Why is the hair around my face darker than the rest of my hair after coloring?
Why is the gray hair around my face turn out lighter then the rest of my hair?
Is it ok if I shampoo my hair a 2nd time after coloring my hair?