Hey !
I'm gonna show you a very easy to make decorative object which uses one single RGB color-changing LED.

Step 1: Materials

The parts list for this project is very small , the only things that you need are :

-A cheap crystal ball , the ones with a flat side which is used to stand still , in our case it's a good space to place the LED in. I got my crystal ball from Ikea .. (Tip: use a ball with bubbles in it , like this one http://farm3.static.flickr.com/2188/2238715683_ff8a6a16f6.jpg , so the LED light can shine in the bubbles)
-An automatically color changing RGB LED (The ones with only two pins , which change their color automatically)
-A Jack male cable from some old headphones
-A Jack female connector
-A battery holder which holds as many batteries as your LED needs (in my case 6V so i used a 4AA battery holder)
-A 5 Liter bottle cap

Tools :

Soldering gun
Hot glue gun

Step 2: Step 1: Put the Led in Place

Get your LED , your 5 L bottle cap and the jack cable

You will see that a headphone jack cable has actually 2 cables glued together , you may want to cut one of the cables as you won't need it.

Remove the protective plastic from one of the two cables and then get a ligher and burn the protective layer from the orange cable , so you will be able to solder on it.

After preparing the cable , solder the two wires to the LED pins , and then use hot glue to put the LED in the center of the cap.

Step 3: Step 2:Glue the Cap to the Crystal Ball

Get your hot glue gun , crystal ball and the cap.

Find the flat side of the ball and place it on the cap , and then use hot glue to keep it in place.
Hot glue is very good for this because if you want the ball back as it was before , just pull the cap very hard and it will come out,leaving no glue marks . Additionally , clean the remaining glue from the ball.

Step 4: Step 3: Polarity Check

Get your jack female connector and remove the plastic part from it , and then plug it into the jack coming from your ball.Now get your power supply's wires and be sure to insert the wires in the plastic part of the jack female connector , i will explain why just a little bit later.
Now use your power supply's wires to check all the 3 connections on the female jack and find the correct ones which will light up the LED . Solder the wires in the correct places on the connector and then use the plastic part to twist it around the connector , so now it's isolated(if you solder without the wires coming out of the plastic isolation , you will have to cut it to put it back).
Now put batteries in your battery holder and it's all done ! The next time you want it to light up just connect the two jacks together.
Now you may notice that i used a 9V battery clip from an old AC/DC adapter for my supply , because my battery holder was using a 9V clip .

Step 5: So How Does It Look Like ?

Just in case you want to see how the finished thing looks like , just look at the posted images and the video .
I hope you all liked my instructable :)