29 year old female have a yellow small bruise on breast is this cancer very worried?

Keep an eye on it: It is unlikely but you should keep an eye for any further changes. Besides looking over the skin may show changes but there is a better way to check for it if you can palpate your breast with the flat part of your hand/fingers and look for a firm to hard mass. It is best done when you are lying down flat in the bed. Painless lump is how breast cancer commonly manifests.