Color individual pixels

Is there a way when rendering a scene to physically change the color of an on-screen pixel or turn it on/off? This is a literal pixel, not a point in world coordinates.

Dogcow “moof!”

Only solution I can think of is after rendering and swap buffer if using double buffer is to have a function to switch to front buffer glDrawBuffer(GL_FRONT) and then switch to ortho mode with it set width and height are equal to width and height of window, then you can overwrite the pixel though you will be using a point to do so.

Only thing I can think of…

fragment program!