Initialise public members of a struct with a default constructor - c++11

Question

Given a toy struct with a default constructor like this one:
struct RGB {
unsigned char r, g, b;
RGB()
:r(0), g(0), b(0) {}
};
How do I initialise one to a specific colour, assuming I don't have access to the source code to add my own constructor.
I don't understand fully why these don't work:
// OK, I can sort-of accept this one
RGB red = {255, 0, 0};
// Not shorthand for green.r=0, green.g=255, green.b=0;?
RGB green = {.r = 0, .g = 255, .b = 0};
// I seem to be missing a constructor that accepts a list?
RGB blue{0, 0, 255};
Is there any other C++11 way to shorten the good old-fashioned:
RGB yellow;
yellow.r = 255;
yellow.g = 255;
yellow.b = 0;
Furthermore, how could I minimally modify the struct declaration to support any of the above, as well as having a default initialisation method.

Answer 1

If you have no possibility to add default arguments to the struct's constructor, how about a helper function:
RGB makeRGB(unsigned char r, unsigned char g, unsigned char b)
{
RGB result;
result.r = r;
result.g = g;
result.b = b;
return result;
}
Which can be used like so:
RGB red = makeRGB(255, 0, 0);
Return value optimization will take care of the temporary and provide a no-overhead solution unless you are using a terrible compiler.
The ideal solution would be modifying the default constructor to take optional arguments:
struct RGB {
unsigned char r, g, b;
explicit RGB(unsigned char r, unsigned char g, unsigned char b)
:r(r), g(g), b(b) {}
RGB() : RGB(0, 0, 0) {}
};
Which can be used like you would expect:
RGB red(255, 0, 0);
RGB green{0, 255, 0};
RGB blue;
blue.b = 255;
Live demo here.

Answer 2

Given
struct color
{
color(std::initializer_list<float> list) = delete;
float r, g, b;
};
color c = {1,2,3} and color c {1,2,3} are the same.
syntax like color c = {.r = 0, .g = 255, .b = 0}; is specific to C programming language, not C++.
color c = {1,2,3}; is perfectly fine in C++11. It is called aggregate initialization. And it does even override explicit constructor from initializer_list. Link describes how to explicitly delete it.
Constructor from initializer_list can be explicitly called by color c({1,2,3})