Fungal infection (onychomycosis): This is the most common cause of toenail discoloration, often resulting in a yellow, brown, or black appearance due to the fungal invasion of the nail.

Trauma: Injury to the nail can cause discoloration, typically presenting as a black or purple color under the nail due to blood accumulation.
Nail psoriasis: This condition can lead to nail discoloration, pitting, and thickening, often with a yellowish or brownish tint.
Aging: As people age, their toenails may become thicker and more prone to discoloration, often taking on a yellowish hue.
Staining from substances: Exposure to certain chemicals or substances (like nail polish) can stain the nails, leading to discoloration.

Melanoma: Although rare, subungual melanoma can present as a dark streak under the nail and is potentially life-threatening if not diagnosed early.
Squamous cell carcinoma: This type of skin cancer can also affect the nail bed, leading to discoloration and other nail changes.
Systemic diseases (e.g., diabetes, liver or kidney disease): Certain systemic conditions can cause nail changes, including discoloration, and it's crucial to identify these underlying diseases.

Yellow nail syndrome: A rare condition characterized by thickened, yellowish nails that are often associated with lymphedema and respiratory diseases.
Argria (silver poisoning): Exposure to silver can cause a bluish-gray discoloration of the nails.
Minocycline-induced nail discoloration: Long-term use of minocycline, an antibiotic, can lead to a blue-gray discoloration of the nails.
Other rare genetic or systemic conditions that may affect nail pigmentation.