Noble Garden

Just to confirm - collect 5 brightly coloured eggs in the noob towns like dolanaar. Open them to get chocolates. When you have 5 chocolates you can buy a Noblegarden Egg from the Noblegarden vendor. Then go to Stormwind. Just right click the egg then click on the ground somewhere to get achievement.

You can then open your own egg to get another chocolate or maybe one of the other better things :)

You can buy a brightly colored egg from the vendor for 5 chocolate. Then use it in Stormwind to place an egg and get the achievement. Then you (or somebody else) can loot the egg and get back one chocolate, and I assume the other items that have a chance to spawn in an egg.

The wording on this achievement is very deceptive. It tell's you to hide a Brightly Colored Egg, which is exactly what you are finding. As already pointed out, you have to hide a Noblegarden Egg.