self.view.backgroundColor =
    [UIColor colorWithRed:123/255.0 green:123/255.0 blue:123/255.0 alpha:1];

#define RGB(r, g, b) 
    [UIColor colorWithRed:r/255.0 green:g/255.0 blue:b/255.0 alpha:1]
#define RGBA(r, g, b, a) 
    [UIColor colorWithRed:r/255.0 green:g/255.0 blue:b/255.0 alpha:a]

self.view.backgroundColor = RGB(123, 123, 123);