Connect one of a soft switch to your Arduino. Use it to control an LED.
Build a circuit with one constructed sensor (variable resistor), your Arduino, and 3 (or more) LEDs or an RGB LED. Use one of the following control structures to turn the LEDs on and off in any sequence when the sensor or switch value changes: if, if/else, while() (e.g. when the sensor goes above a threshold, the LED turns one color; when it goes above another threshold, it changes to a second color, etc).
Document both of these to the blog with images and your code.