Eccentric structureless area any colour except skin

Structureless areas are only a weak clue to melanoma as they can be seen in a wide variety of lesions both melanocytic and non melanocytic. 

Structureless and brown may be seen in skin areas with flattened rete ridges. If they were prominent the pattern would be reticular!  With structureless areas try to exclude solar lentigo and flat seborrhoeic keratoses early. 

Structureless and brown can also be a pigmented IEC. 

Structureless white areas usually represent regression in melanomas. Often grey dots are associated.

Regression in a solar lentigo 

Structureless and black means melanoma or Clark or Reed nevus but can also represent old blood or a thrombosed hemangioma.

Thombosed angiokeratoma

Structureless and blue is a feature of a blue nevus but also some foreign bodies and melanoma metastases.

A grey structureless centre to a lesion usually means acanthosis (thickening) of the epidermis and some hypergranulosis.(Kittler)

A red structureless area usually means fresh blood.
Remember you can have vessels in structureless areas.