I suggest you make a skybox: A skybox is a large cube which you render around your camera which will give you the ability to pass each pixel through a fragment shader. And you can render anything onto the skybox: a color, an image, a gradient, etc.
Just be sure to call glClear(GL_COLOR_BUFFER_BIT || GL_DEPTH_BUFFER_BIT);
A good tutorial on this topic is this one: http://antongerdelan.net/opengl/cubemaps.html