pregnancy, hormonal changes or medications can also cause yellow nipple. but let's not forget breast cancer also can be one of the symptoms.
Nipples: A Sensitive Topic
8 ways your nipples can change when you breastfeed - Today's Parent
Quality at a Steal Nipple Color and Other Changes: What's Normal?, nipple pain
What To Know About Leaky Nipples in Pregnancy
Causes of Nipple Discharge - Women's Health Network
Nipple Darkening during Pregnancy: Causes & Tips to Deal with It
Galactorrhea: Causes, Symptoms & Treatment
Why Did the Color of My Breast Milk Change?
Breast swelling
8 Weird Things That Happen to Your Breasts When You're Pregnant
How to Relieve Breast Pain and Sore Nipples During Pregnancy