Since I found this library, I feel very easy to use. I often use this library to implement some small programs.
It's not a shame to declare yourself as the author of the library
class ColorEx : public sf::Color { .. }
Your class has no member variables and member functions, but acts as a pure namespace. Neither inheritance nor a class is needed,
namespace is what you are looking for.
Also, I would not use the namespace
sf for extensions to SFML.
Nevertheless, this can be a useful collection of colors!