Okay, licensed professional here.... This is the scoop.
When you pick a color from the store that says cool or ash tones it will have a green base to it. This is best for someone who already has a lot of warm or reddish copper tones that they want to get rid of.
When you pick a color that says warm or golden, it has a red base to it. This is if you want the reddish tones in.
So, picking a color is the most important part of coloring. Take your time reading the box. Determine what your natural color is. If it's a warm brown and you want to keep that, don't pick a cooler or ash color, pick one that says golden or warm. Usually there is a color scale on the side of the box.
It's fine to re-color your hair within 24 hours. Just make sure it's fully dry from washing. If you like the tone, but it's just too dark, mix a couple tablespoons of peroxide with your shampoo, let it sit for about 5-10 minutes. Wash, condition and re-dry.
If you are totally unhappy with the tone, too red (warm or golden) or too ash (cool or greenish), pick a new color with appropriate tone you need to counteract what you have.
Hope that helps, but if you are totally lost, go to a quality salon.
PS - I worked in a salon for years, and still use color in a box.
