In the context of IP law, “significance” is measured by whether a feature is essential to an article’s use or purpose.

The functionality doctrine, as we have said, forbids the use of a product's feature as a trademark where doing so will put a competitor at a significant disadvantage because the feature is "essential to the use or purpose of the article" or "affects [its] cost or quality." Inwood Laboratories, Inc., 456 U. S. Reports 850 , n. 10, 102 S.Ct., at 2186 , n. 10. The functionality doctrine thus protects competitors against a disadvantage (unrelated to recognition or reputation) that trademark protection might otherwise impose, namely their inability reasonably to replicate important non-reputation-related product features. For example, this Court has written that competitors might be free to copy the color of a medical pill where that color serves to identify the kind of medication ( e.g., a type of blood medicine) in addition to its source. See id., at 853, 858, n. 20, 102 S.Ct., at 2188 , 2190, n. 20 ("[S]ome patients commingle medications in a container and rely on color to differentiate one from another"); see also J. Ginsburg, D. Goldberg, & A. Greenbaum, Trademark and Unfair Competition Law 194-195 (1991) (noting that drug color cases "have more to do with public health policy" regarding generic drug substitution "than with trademark law"). And, the federal courts have demonstrated that they can apply this doctrine in a careful and reasoned manner, with sensitivity to the effect on competition. Although we need not comment on the merits of specific cases, we note that lower courts have permitted competitors to copy the green color of farm machinery (because customers wanted their farm equipment to match) and have barred the use of black as a trademark on outboard boat motors (because black has the special functional attributes of decreasing the apparent size of the motor and ensuring compatibility with many different boat colors). See Deere & Co. v. Farmhand, Inc., 560 F.Supp. 85 , 98 (SD Iowa 1982), aff'd, 721 F.2d 253 (CA8 1983) ; Brunswick Corp. v. British Seagull Ltd., 35 F.3d 1527 , 1532 (CA Fed.1994), cert. pending, No. 94-1075; see also Nor-Am Chemical v. O.M. Scott & Sons Co., 4 U.S.P.Q.2d 1316 , 1320, 1987 WL 13742 (ED Pa.1987) (blue color of fertilizer held functional because it indicated the presence of nitrogen). The Restatement (Third) of Unfair Competition adds that, if a design's "aesthetic value" lies in its ability to "confe[r] a significant benefit that cannot practically be duplicated by the use of alternative designs," then the design is "functional." Restatement (Third) of Unfair Competition § 17, Comment c, pp. 175-176 (1995). The "ultimate test of aesthetic functionality," it explains, "is whether the recognition of trademark rights would significantly hinder competition." Id., at 176.

The upshot is that, where a color serves a significant nontrademark function—whether to distinguish a heart pill from a digestive medicine or to satisfy the "noble instinct for giving the right touch of beauty to common and necessary things," G.K. Chesterton, Simplicity and Tolstoy 61 (1912)—courts will examine whether its use as a mark would permit one competitor (or a group) to interfere with legitimate (nontrademark-related) competition through actual or potential exclusive use of an important product ingredient. That examination should not discourage firms from creating aesthetically pleasing mark designs, for it is open to their competitors to do the same. See, e.g., W.T. Rogers Co. v. Keene, 778 F.2d 334 , 343 (CA7 1985) (Posner, J.). But, ordinarily, it should prevent the anticompetitive consequences of Jacobson's hypothetical "color depletion" argument, when, and if, the circumstances of a particular case threaten "color depletion."