Sorry you're having difficulties. What step exactly in the tutorial is causing the problems? At a guess, I'd say perhaps you have a layer set to a blend mode other than Normal (perhaps Overlay or Multiply)? If the background colour changes, sometimes layer blends won't show. Or have you accidentally merged the wrong layers?

About the mouse/tablet issues, Gimp treats them as two separate inputs and remembers the last settings for each. So, for example, if you pick a red colour with your mouse, then pick up your tablet pen, the colour will change to whatever colours you had when you last used the pen. Put the pen down and go back to the mouse, it'll bring back the red colour you picked earlier. This is how Gimp handles things and I don't think you can change it (some people prefer different settings for each device, I guess.) It's best to get into the habit of choosing your settings while using the tablet pen. If you do that, you shouldn't have any problems.