If dandruff flakes are greasy and yellow, the probable cause is the skin condition known as seborrheic dermatitis; seborrhea is usually associated with redness as well. Dry, thick lesions consisting of large, silvery scales may be traced to the less-common psoriasis of the scalp. These scaly conditions become a hazard only if you scratch to the point of causing breaks in the skin, which can place you at greater risk for infections, particularly from staph and strep bacteria. Read more about dandruff.
Slideshow: Slideshow: What Your Hair & Scalp Say About Your Health
Article: Understanding Dandruff - the Basics
Article: Understanding Dandruff - Symptoms