I made a color-matching game called HueHunter. Colored blocks bounce around the screen, and I have to adjust my own color before shooting them. Being able to click the block is only half the problem.
The code keeps red, green, and blue values separately. The color buttons change those components; a hit only removes a block if all three differences are within the allowed range. When a level is cleared, the next one adds more blocks and increases their speed.
It is another AWT applet with mouse input, sounds, and double-buffered drawing. The moving blocks have their own position, velocity, and color values, while the main applet handles the round and the player controls. I like the idea of using the actual RGB values as the game rule instead of just using color to make ordinary targets look different.