colours so iconic that you could end up in court for using

Barbie Pink (Pantone 219C)

While not legally trademarked, Mattel has used the Pantone 219C colour so much it has gained a second meaning and is often associated with the Barbie brand, giving it the right to be associated to Mattel thru common law channels, especially within the United States and in their law which could lead to legal proceedings if the colour is used in other works without Mattel authorisation as this can be challenged in court.

Cadbury Purple (Pantone 2685C)

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The iconic chocolate brand has managed to trademark the purple used on all their products as it has been used for years, similarly to Mattel and has been associated with the Cadbury brand, however upon the trademark, the judge in charge clarified that just because Cadbury had managed, it does not mean everyone should as they will likely get turned down unless as recognisable as the the Cadbury Purple. [Source]

Louboutin Red (Pantone 18-1663 TPX)

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The iconic red sole of the Louboutin has been seen over and over within the media and all over tabloids, being worn and endorsed by big celebrities, therefore the designer argued in court that the red sole had gained a secondary meaning to the consumers, which as seen with Mattel, within US law is a valid point to make when fighting to trademark a colour. However the appeal failed and the court refused to accord trademark to the design company, however, they did gain trademark over using the red color on the sole of a heeled shoe, especially if that colour isn't the only one present within the shoe, so this one isnt exactly a trademarked colour but it sure can be if mixed with other elements, and can get you summouned in court. [Source]

I didn't even know some of these were under a trademark, and I knew even less about secondary meaning and how a colour could essentially act as a logo, there are plenty of other colours I could discuss but with how many established brands there are I would be here all day.