“Colour marks” are registered trade marks consisting of a single colour, used in abstract, as opposed to, say, being part of a logo. Big businesses (and they usually are big) like to register colour marks as a way of extending their brand protection and maximising control over their house colour schemes. So Barclays have their “Barclays Blue”, BP has its particular shade of green etc.
In theory, provided you satisfy the basic trade mark application requirements, colour marks are perfectly permissible. The trouble is, there aren’t that many colours to go round, and you run into trouble if you start allowing people to monopolise them.