Scalp psoriasis and seborrheic dermatitis are two skin conditions that cause rough, scaly skin on the scalp. Psoriasis can develop anywhere on the body, while seborrheic dermatitis primarily affects the scalp and appears as yellow, greasy scales with red skin. Treatment options for scalp psoriasis include topical corticosteroids and light therapy, whereas seborrheic dermatitis often improves with over-the-counter shampoos.