Things to check for: make sure the new layer is on top, make sure the layer's opacity is 100% (for now, you can change it later), make sure the layer has normal blend mode, make sure that the brush's opacity is 100% (for now, you can change it later), make sure that the brush's blend mode is normal, make sure that the brush doesn't have any other characteristics that you don't understand (like spacing, texture, dual brush, fade, color dynamics, scatter, etc). If you give us a screenshot of the whole screen (not just the map) with the troublesome layer active then we should be able to figure it out real quick. To make a screenshot just press the Print Screen button on your keyboard. This saves the image into memory, then just hit File - New and leave the settings alone, and then hit Edit - Paste and voila there's your screen. Now hit Layer - Flatten Image and save it as a jpg and upload it.