Brassy hair is normally hair that has been dyed and not gone through all of the stages of color development. Human hair has many levels and undertones. Dying hair red or blond can cause a yellow or orange color to appear.
Think of a new copper penny and how it shines in the light. Brassy hair does the same thing. It doesn't look natural. To fix brassy hair, it's best to use a toner to remove the brassy appearance. There are brass removers, but toning works much better and won't leave hair dull and faded. Before purchasing a toner, it's best to know if hair has cool or warm undertones. It also helps when bleaching hair to do so with a purple- or blue-toned bleach to avoid brassy hair.