Oh fer...
No ITs NOT legal to copyright a color.
But that doesn't stop corporations from claiming that they have.
Because the government does not check copyright filings for accuracy or prior art, you can file a copyright for anything.
Corporate lawyers do a lot of things that are illegal. For example, in most states, most non-disclosure agreements are illegal. ( if they don't pay you something... its not a legal contract)
If you take this kind of thing to court, you will ALWAYS win... all you have to do is show that that particular color appeared in internet publications previously to the companies' use of it....
They can trademark the combination of a color with a specific word, letter phrase or graphic... but in that case its only that particular use in a competing field.
The standard of proof is that the company has to be able to show in court that you are trading on their reputation. That is either confusing customers as to the company identity, or capitalizing on their market presence.
The real crime is that these corporation know full well they haven't a legal leg to stand on... but are relying on financial intimidation to get folks to back down without going to court. They can run you into tens of thousands of dollars in legal fees, and drop the case the day before the court date and you have no legal recourse to re-coup expenses.
I have gotten letters from the family of Martin Luther King claiming that they own the rights to his likeness...
They don't- you can't own the likeness of a public figure... and anyone with streets and a holiday named after them is clearly a public figure...
But no one has ever fought the family on the issue... they just back down in the face of high legal costs.
So, no, you can not legally enforce a copyright of an individual color, alone. But they CAN sue you over it to cost you money.
However, if you change ONE of the color channels, R G or B by one percent... Problem solved...
Or- I have found that a very stiffly worded letter in response, explaining the legal issue, and your absolute willingness to not only fight them at any cost, but to sue them for abuse of process, and filing an illegal copyright... is often enough to get them to ignore you.
Where they will tend to be most aggressive is in any usage that might be seen as trading on their image, or confusing the marketplace...
But the overarching lesson is this... a conservative estimate that half of all contracts written in the US would not hold up in court.
It is astonishing how often lawyers include clauses in contracts that they know full well can not be enforced... they just want you to believe they can be, or want the option to sue you simply as a punitive action.