A very small piece of advice which I don't see mentioned here: OpenGL works in linear RGB (which is very convenient), while your monitor displays sRGB (which gives you better colour resolution due to how eyes work). This means that outputting the result of OpenGL directly on your screen is wrong (a lot dimmer than it should be). Use this little fragment in all your shaders to convert the final output to sRGB:

    vec3 srgb(vec3 rgb) {
        vec3 mask = vec3(greaterThan(rgb, vec3(0.0031308)));
        return mix(rgb * 12.92,
                   pow(rgb, vec3(1.0 / 2.4)) * 1.055 - 0.055,
                   mask);
    }

Edit: Here's a very good example of the difference https://imgur.com/a/g6qNI

Edit 2: And of course when you load textures you must tell OpenGL they're in sRGB colourspace (most probably they are, if you created them on a normal monitor), so it will do the reverse transformation when reading from the texture. All this trouble could have been avoided if monitors were 30bit linear RGB colourspace to begin with, but that's history now. C'est la vie.