Anyone have an example of a Brightness Slider shader?

Hi everyone, I've been using gamemaker for a while now but I'd never felt the need to use shaders. That is, until now. I'm aware that a crude method of implementing a 'Brightness' setting WITHOUT using shaders would be as simple as drawing a black square over the screen and changing its alpha. But ideally I need something that would allow me to increase/decrease the colour values of my sprites beyond/below the values they hold by default. So just a basic brightness slider, really. Does anyone have an example of how to do this?

Thanks in advance to anyone who provides an answer.