As far as logos go, few are as iconic and instantly recognizable as Apple's old-school rainbow emblem. No matter how sleek their products get, people are still nostalgic for the old, colorful logo, and this mod from Adafruit basically proves it. If you have some pretty decent soldering skills, you can make the Apple on the back of your iPhone light up like the old rainbow logo.
In the video below, Becky Stern from Adafruit uses a kit with a replacement glass back, tweezers, screwdrivers, and decals to add the color. This will only work with an iPhone 4S or earlier because the new one doesn't have an all-glass back. It also, not surprisingly at all because this is Apple we're talking about, totally voids your warranty, so proceed with caution.
After removing the back of the phone, Becky removes the screw for the power connection cable and pulls the connection apart to make sure there's no power. Then, she removes the shield and protectors from the circuits at the top. Using a USB microscope (those components are tiny!), she solders everything in place, sticks a colorful decal over the Apple logo, and puts everything back together.
Besides soldering those teeny tiny little wires, the hardest part about this mod is probably tracking down the parts. Adafruit doesn't sell the kits because they come with a trademark-infringing replacement back, but you should be able to track one down online and you can buy everything else you need on their website.