Although both seborrheic dermatitis and psoriasis are common conditions that affect the skin, they have slightly different symptoms. Psoriasis tends to display thicker scales on the scalp than seborrheic dermatitis, and it typically affects multiple parts of the body such as the elbows, knees, hands, or feet.Seborrheic dermatitis tends to manifest itself as red skin covered with greasy-looking white or yellow scales, whereas psoriasis scales tend to be flaky.