shade (n.)

Middle English shade, schade, Kentish ssed, "dark image cast by someone or something; comparative obscurity or gloom caused by the blockage of light," from late Old English scead "partial darkness; shelter, protection," also partly from sceadu "shade, shadow, darkness; shady place, arbor, protection from glare or heat." Both are from Proto-Germanic *skadwaz (source also of Old Saxon skado, Middle Dutch scade, Dutch schaduw, Old High German scato, German Schatten, Gothic skadus), from PIE *skot-wo-, from root *skoto- "dark, shade." 

shade, shadow, nn. It seems that the difference in form is fairly to be called an accidental one, the first representing the nominative & the second the oblique cases of the same word. The meanings are as closely parallel or intertwined as might be expected from this original identity, the wonder being that, with a differentiation so vague, each form should have maintained its existence by the side of the other. [Fowler]

Figurative use in reference to comparative obscurity is from 1640s. Hence throw into the shade, etc., "obscure by contrast or superior brilliancy." The meaning "a ghost" is from 1610s; dramatic (or mock-dramatic) expression shades of _____ to invoke or acknowledge a memory is from 1818, from the "ghost" sense. Meaning "lamp cover" is from 1780. Sense of "window blind" is recorded by 1845. The meaning "cover to protect the eyes" is from 1801. Meaning "grade of color" is recorded from 1680s; that of "degree or gradation of darkness in a color" is from 1680s (compare nuance, from French nue "cloud"). Meaning "small amount or degree" is from 1749.

c. 1400, shaden, "to screen from the sun or its heat," from shade (n.). From 1520s as "to cast a shadow over;" the figurative use in this sense is from 1580s. The meaning in painting and drawing is from 1797. In reference to colors, 1819. Related: Shaded; shading.