int _tmain(int argc, _TCHAR* argv[])
{
if (SDL_Init(SDL_INIT_EVERYTHING) != 0){
std::cout << "SDL_Init Error: " << SDL_GetError() << std::endl;
return 1;
}

SDL_Window *win = SDL_CreateWindow("Hello World!", 100, 100, 640, 480, SDL_WINDOW_SHOWN);

if (win == nullptr){
std::cout << "SDL_CreateWindow Error: " << SDL_GetError() << std::endl;
return 1;
}

SDL_Renderer *ren = SDL_CreateRenderer(win, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
if (ren == nullptr){
SDL_DestroyWindow(win);
std::cout << "SDL_CreateRenderer Error: " << SDL_GetError() << std::endl;
SDL_Quit();
}

SDL_Color clr1 = { 255, 0, 0, 255 };
SDL_Color clr2 = { 0, 0, 255, 255 };

float _dr = clr2.r - clr1.r;
float _dg = clr2.g - clr1.g;
float _db = clr2.b - clr1.b;

float __dr = 0;
float __db = 0;
float __dg = 0;

_dr /= 600;
_dg /= 600;
_db /= 600;

for (int i = 0; i < 600; i++)
{

__dr += _dr;
__dg += _dg;
__db += _db;

SDL_SetRenderDrawColor(ren, clr1.r + __dr, clr1.g + __dg, clr1.b + __db, 255);

SDL_RenderDrawLine(ren, 0, i, 800, i);
}

SDL_RenderPresent(ren);

SDL_Delay(2000);

SDL_DestroyRenderer(ren);
SDL_DestroyWindow(win);
SDL_Quit();

_getch();

return 0;
}

int SDL_SetRenderDrawColor(SDL_Renderer* renderer,
                      Uint8         r,
                      Uint8         g,
                      Uint8         b,
                      Uint8         a)

skyColorTop.r +(int) __dr

(int) __dr

color _clr = RGB(clr1.R+dr,clr1.G+dg,clr1.B+db)

SDL_Color skyColorTop={255,0,0,0};            
SDL_Color skyColorBottom={0,0,255,255};