Excluding red and skin colors we will look at lesions that are an uncommon colour.
Yellow lesions Yellow lesions look yellow because of fat, sebaceous material, carotene, jaundice pigment or drugs. At one stage resolving bruises go through a yellow phase. Yellow papules or nodules are Xanthomas, xanthogranulomas including necrobiotic, Sebaceous hyperplasia and other sebaceous tumours, Gouty tophi around elbows and knees.
Yellow skin consider carotene pigmentation of palms and soles, jaundice from any cause and drugs such as quinacrin.
Purple lesions
Purple lesions are that colour because of altered or venous blood , overgrowth of blood vessels or infiltrates of neutrophils or lymphocytes into the skin. Consider therefor haemangiomas, angiosarcomas, Kaposi's sarcoma, port wine stains or other vascular malformations, vasculitis with leaked blood cells, lymphocytoma, lymphoma, and the plaques and nodules of Sweet's syndrome where the infiltrating cells are lymphocytes. Sarcoidosis can also have a purple colour in the skin. Better also consider drugs giving a lichenoid drug reaction eg Thiazides and of course Lichen planus itself with itchy papules at the wrists and lower legs.
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Green lesions
There are not many green lesions in the skin. Copper bracelets can cause green staining under them. Thereafter it would need to be exogenous pigmentation from a dye. Do not know any drugs causing a generalised green skin colour.
Blue lesions
Things appear blue in the skin if melanin is in the upper dermis or below. It has something to do with light scattering by the tissues. The same melanin higher in the epidermis shows as black. Venous blood is also blue as is the drug minomycin or clofazimine when they are deposited in the skin. Blue papules are blue nevi , melanoma metastases, leukaemic deposits or some lichen planus papules. Blue nodules in neonates are due to severe viral infections with extramedullary haematopoesis or neuroblastoma metastases!
Cyanosis makes you blue as well!
Grey lesions
Again consider drugs such as Minocycline and post inflammatory hyperpigmentation particularly with fixed drug reactions and lichen planus. It is usually due to melanin in melanophages in the dermis.
