http://www.histopathology-india.MelanocyticTumours.htm

This dermal melanocytic lesion presents as slate coloured patches of discolouration.

Mongolian spots are usually seen in Orientals. It is present at birth or appears in early infancy and usually disappears in childhood. 

Site:  Located at the sacrococcygeal region as well as in the scalp and temple.

Microscopic features:  Histopathology reveals scattered slender melanin containing  melanocytes orientated horizontally in the deep part of the reticular dermis together with a few melanophages.
                                 Image Link

These dermal melanocytic conditions present as blue to dark brown macular area of pigmentation .

The  nevus of Ota  is usually located in the  region of opthalmic and maxillary division of the trigeminal nerve. The conjunctiva may also be involved. Ota's nevus may be associated with oral, uveal or leptomeningeal melanosis and malignant transformation has been reported repeatedly.

The nevus of Ito is usually located in the supraclavicular and deltoid region.

Histologically there are nodular and diffuse collection of melanocytes. These are scattered in the upper parts of the reticular dermis.  The appearances resemble those of blue naevus.

In contrast to Mongolian spot these hyperpigmentations do not disappear.
      
    Nevus of Ito-     Image Link1 ;  2

   Nevus of Ota-    Image Link
 

Prognostic parameters of melanoma

Lentigo maligna melanoma

Superficial spreading melanoma

Nodular melanoma

Acral lentiginous melanoma

Desmoplastic /Spindle cell /
Neurotropic melanoma

Naevoid melanoma

Balloon cell melanoma