inline Colour operator /(const Colour& a, double s) { return Colour(a.R()/s, a.G()/s, a.B()/s, a.A()/s); }
inline Colour operator +(const Colour& a, const Colour& b) { return Colour(a.R()+b.R(), a.G()+b.G(), a.B()+b.B(), a.A()*b.A()); }
inline Colour operator *(double s, const Colour& a) { return Colour(s*a.R(), s*a.G(), s*a.B(), s*a.A()); }