Discoid Lupus

DLE include loss of pigmentation where the brown/black color of the nose changes to a slate blue or pink color, redness, and scaling of the nose. The disease can ultimately progress to significant destruction of the tissue, resulting in ulceration and crusting of the nose. 

Reactive Histiocytosis

Cutaneous histiocytosis:  Lesions typically are multiple, red plaques or nodules that occur anywhere on the body.  Nodules may be ulcerated, but are not usually painful or itchy. Some dogs have lesions that remain confined to the nose, resulting in a “clown-nose” appearance.  Nodules may wax and wane, or regress and then appear elsewhere on the body. 
Systemic histiocytosis:  Clinical signs vary with the severity of the disease.  The cutaneous signs can be very similar to cutaneous histiocytosis, but nodules can involve the eyelids and nasal mucosa as well.