The range of sensors that can be used with the Arduino and other development platforms is constantly growing in both variety and usefulness. One example of this is a colour sensor - something that was quite expensive in the past but now much more affordable. These will reflect light from an object and measure the reflected light levels, converting this into data that can be analysed.
These sensors have been demonstrated in a fun example by MarkusB in the LMR forum, who demonstrates using a TCS3200 colour sensor with an Arduino by creating a chameleon-like device that uses an RGB LED to relay the colour being measured - as shown in the following video:
This is a fun project that isn't difficult to replicate - simply visit the project page for the schematic and required sketch. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're looking to make 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.