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.