Companies successfully trademarked combinations of colours, but the US Patent and Trademark Office dismissed attempts to trademark a single colour. 

Arguments against issuing single colour trademarks:

• Colour depletion theory: Only around 1,867 solid Pantone colours exist. We'll run out if brands all claim a colour.
• Shade confusion theory: It would be hard for consumers to differentiate between slight shade variations of colours claimed by brands.

In the late 1950s, Owens-Corning was facing steep competition from other fibreglass insulation companies.  Owens-Corning decided to distinguish itself by infusing its product with pink dye. The company adopted the slogan "think pink" and spent tens of millions of dollars advertising the colour.

In 1985, after a 5-year battle, Owens-Corning became the first company in American history to successfully trademark a colour. Ten years later, the Supreme Court ruled that colour could serve to identify a brand.

A number of companies a have trademarked single colours. 

• In 1998, Tiffany & Co. trademarked its famous blue, and UPS trademarked its “Pullman Brown.”
•  3M secured its signature canary yellow colour for its Post-it notes.
• Deutsche Telekom AG protected T-Mobile’s famous magenta, and Fiskars has one for orange scissor handles.
• The Wiffle Ball, Inc. has a trademark on yellow for use in bats.
• The University of Texas at Austin (Pantone 159) and The University of North Carolina at Chapel Hill (Pantone 542) both have protections on their school colours.

A company with a colour trademark only “owns” the colour in connection to particular goods or services.

To successfully secure such a trademark, a firm must prove that a single colour:

• Achieves “secondary meaning” (distinguishes a product from competitors and identifies the company as the definitive source of the product)
• It doesn’t put competitors at a disadvantage by affecting cost or quality.
• It doesn’t serve a functional purpose.

Brands will go through the trouble of trademarking a colour when it is critical to the band, sales, or how the product is marketed.

When a company files a trademark in black and white — the trademark is protected in all colour variations by default. So nobody can take the McDonald’s red and yellow logo, make it purple and green, and claim it as his own.