"343 Neither can we find a principled objection to the use of color as a mark in the important “functionality” doctrine of trademark law. The functionality doctrine prevents trademark law, which seeks to promote competition by protecting a firm’s reputation, from instead inhibiting legitimate competition by allowing a producer to control a useful product feature. It is the province of patent law, not trademark law, to encourage invention by granting inventors a monopoly over new product designs or functions for a limited time, 35 U.S.C. §§ 154, 173, after which competitors are free to use the innovation. If a product’s func tional features could be used as trademarks, however, a monopoly over such features could be obtained without regard to whether they qualify as patents and could be extended forever (because trademarks may be renewed in perpetuity). See Kellogg Co. v. National Biscuit Co., 305 U.S. 111, 119-120, 59 S.Ct. 109, 113-114, 83 L.Ed. 73 (1938) (Brandeis, J.); Inwood Laboratories, Inc., supra, 456 U.S., at 863, 102 S.Ct., at 2193 (White, J., concurring in result) (“A functional characteristic is ‘an important i ngredient in the commercial success of the product,’ and, after expiration of a patent, it is no more the property of the originator than the product itself”) (citation omitted). Functionality doctrine therefore would require, to take an imaginary example, that even if customers have come to identify the special illumination-enhancing shape of a new patented light bulb with a particular manufacturer, the manufacturer may not use that shape as a trademark, for doing so, after the patent had expired, would impede competition – not by protecting the reputation of the original bulb maker, but by frustrating competitors’ legitimate efforts to produce an equivalent illumination-enhancing bulb. See, e.g., Kellogg Co., supra, 305 U.S., at 119-120, 59 S.Ct., at 113-114 (trademark law cannot be used to extend monopoly over “pillow” shape of shredded wheat biscuit after the patent for that shape had expired).\n\n344 This Court consequently has explained that, ”in general terms, a product feature is functional,” and cannot serve as a trademark, “if it is essential to the use or purpose of the article or if it affects the cost or quality of the article,” that is, if exclusive use of the feature would put competitors at a significant non-reputation-related disadvantage. Inwood Laboratories, Inc., 456 U.S., at 850, n. 10, 102 S.Ct., at 2186, n. 10. Although sometimes color plays an important role (unrelated to source identification) in making a product more desirable, sometimes it does not. And, this latter fact – the fact that sometimes color is not essential to a product’s use or purpose and does not affect cost or quality – indicates that the doctrine of “functionality” does not create an absolute bar to the use of color alone as a mark. See Owens-Corning, 774 F.2d, at 1123 (pink color of insulation in wall “performs no nontrademark function”). It would seem, then, that color alone, at least sometimes, can meet the basic legal requirements for use as a trademark. It can act as a symbol that distinguishes a firm’s goods and identifies their source, without serving any other significant function. See U.S. Dept. of Commerce, Patent and Trademark Office, Trademark Manual of Examining Procedure § 1202.04(e), p. 1202-13 (2d ed. May, 1993) (hereinafter PTO Manual) (approving trademark registration of color alone where it “has become distinctive of the applicant’s goods in commerce,” provided that “there is no competitive need for colors to remain available in the industry” and the color is not “functional”); see also 1 McCarthy §§ 3.011, 7. 26 (“requirements for qualification of a word or symbol as a trademark” are that it be (1) a “symbol,” (2) “used … as a mark,” (3) “to identify and distinguish the seller’s goods from goods made or sold by others,” but that it not be “functional”). Indeed, the District Court, in this case, entered findings (accepted by t"