There are no such settings. You can only change the brightness and enable/disable power saving mode.
This is what power saving mode does:
Power-Saving Mode increases battery life by automatically adjusting the Wii U GamePad's screen brightness and color based on what is being displayed at the time.
You can't change the sharpness, color/tone, or contrast as you can on a TV typically. Your best bet is to disable power saving mode and crank up the brightness to your liking.
EDIT: It's actually
impossible to have better dynamics (contrast, color) on the Wii U GamePad considering the Wii U itself operates in limited RGB and compresses the picture to YUV 4:2:0 colorspace (losing any and all details there are in the color information). Add to the fact that the Wii U GamePad uses a low quality screen that's not designed for rich/vibrant color reproduction. Viewing angles alone can tell how good the quality is on a display and the Wii U GamePad is definitely using older technology than IPS LCD that's used in most gadgets, which also has far better color reproduction