C++ How to design a class that can take on only a discrete set of values, e.g. Red Green Blue

Say for example that we want to design a color class, assuming that we only allow for 3 different color values: Red, Green, Blue.

How is this generally achieved in C++? Or better what is the general design pattern for this?

The only way I could think of, is to initialize 3 different static colors in the Color class,, make the colourCode inaccessible and allow construction of a new Color only with an already existing one.

class Colour
{
public:
    static const Colour Red;
    static const Colour Green;
    static const Colour Blue;

    Colour(){}

    bool operator==(const Colour &other) const{
        if(other.colourCode == this->colourCode){return true;}else{return false;}
    }

    Colour & operator=(const Colour & other){
        this->colourCode = other.colourCode;
    }

    Colour(const Colour & other){
        *this = other;}

private:
    int colourCode;

    Colour(int colourCodeIn){
        colourCode = colourCodeIn;
    }
};

const Colour Colour::Red(1);
const Colour Colour::Green(2);
const Colour Colour::Blue(3);

class Pen
{
public:
    Pen(){}
    void setColour(const Colour & infarbe){
        farbe = infarbe;
    }
    Colour farbe;
};

int main()
{
    Pen myPen1;

    myPen1.setColour(Colour::Blue);

    return 0;
}