the compound adjective 'steel-blue' is a straightforward descriptive formation in english, following the common pattern of noun+adjective to specify a particular shade. the first element, 'steel', borrowed from the old english term for the metal alloy, serves as a metaphorical reference point, evoking the cool, greyish, and often slightly lustrous hue characteristic of polished steel. the second element, 'blue', of germanic origin via old french, provides the core colour category. this type of compounding allows for precise colour designation by comparing it to a well-known material, a process common in the development of english colour terminology, especially during periods of industrial and material familiarity.

Structure: steel (steel) + blue (blue) 'steel' (from old english 'stēle', a hard, strong alloy of iron and carbon; used here attributively to denote a quality or resemblance). 'blue' (from old french 'bleu', of a colour intermediate between green and violet, like the sky on a clear day; the head noun denoting the colour).