You can use a library like https://imagemagick.org/script/magick++.php to complete the task:
#include <Magick++.h>
Magick::Color color(red, green, blue);
Magick::Geometry size(width, height, /*xOff=*/0, /*yOff=*/0);
Magick::Image image(size, color);
image.write( "image.jpg" );
See https://www.imagemagick.org/Magick++/Image++.html for some more examples.