Pityriasis Versicolor: This condition is characterized by patches of discolored skin that can appear grey, along with itching. It's caused by a yeast infection and is common in young adults, making it a likely diagnosis for someone presenting with grey scales and itchy skin.

Eczema (Atopic Dermatitis): Eczema can cause itchy, inflamed skin that may appear grey or discolored due to chronic scratching or inflammation. It's a common condition that can present at any age.
Psoriasis: While typically associated with red, scaly patches, some forms of psoriasis, like inverse psoriasis, can present with greyish scales and significant itching.
Seborrheic Dermatitis: This condition causes scaly patches, red skin, and stubborn dandruff, often accompanied by itching. It can affect oily areas of the body, like the face, but can also appear on other parts of the body.

Cutaneous T-Cell Lymphoma (Mycosis Fungoides): Although rare, this is a type of skin cancer that can present with patches of discolored, itchy skin. Missing this diagnosis could have severe consequences, making it crucial to consider, especially if other symptoms like lymphadenopathy are present.
Hypopigmented Mycosis Fungoides: A variant of cutaneous T-cell lymphoma that can present with hypopigmented patches, which might appear grey, and is accompanied by itching.

Pityriasis Alba: A mild form of eczema that causes small patches of light skin, often with fine scales, typically on the face. It's more common in children and can be itchy.
Grey Pigmentation in Arsenic Poisoning: Chronic arsenic exposure can lead to grey or pigmented skin lesions, among other symptoms. This is rare and usually associated with a history of exposure to arsenic.
Sjögren's Syndrome: An autoimmune disorder that can cause skin dryness and itching, potentially leading to greyish appearance due to scaling, though this is less common and usually accompanied by other systemic symptoms like dry eyes and mouth.