Controlling Arduino-connected devices via Android is certainly possible however not for the absolute beginner (though don't let that stop you!). In some cases you may just want a specific task - such as controlling a light. In this example you can use a free or paid Android app to control a Bluetooth-equipped Arduino board - which then controls individual red, green and blue LEDs for various lighting effects. The code is available under a GPL3 open-source licence, and the results are very good - for example:
If you're looking to make more creative designs with LEDs, consider the Freetronics RGBLED: full colour module. It includes a bright RGB LED on the top of the board and a WS2801 constant-current, addressable, multi-channel LED driver on the back. This smart module can be daisy-chained, so you can connect a number of these together in a string and drive each of the module colours individually from your microcontroller. For more information and to order, visit the product page.