why aren't my brushes working

For some reason my brushes simply aren't working... I am trying to pencil in individual pixels on a background image that is unlocked and for some reason nothing I do is causing any sort of new coloration to appear... I have accidentally caused a color change twice but I do not know how I did it... this is the layout I am currently dealing withPlease can somebody throw me a line, I will have to use this technique on this image 55 times until I get the needed results (I will need to be able to do it on my own) and as of now I can't get it to do a thing... I am fairly new to Photoshop but I desperately need to be able to draw on this gauge pixel by pixel... I already know how to re-size my brush and many other things... but I don't know why it's not working... SOS!!!