Yes, I completely agree with Twinergy!  Being a music teacher, I found that perfect pitch be trained.  Like Twinergy mentioned, if you have heard and sing (hum) the "sound" for a zillion times, you trained your brain to "memorize" the "sound" of a particular pitch. 

In my many years of teaching, over 70% of my students have developed perfect pitch and the rest have excellent relative pitch.  This training is done at a very young age through lots and lots of singing. 

I am sooo surprise to read about this "color system" in learning piano!!  The first time I came across this method is at My Musikbox, they offer a piano program called First Touch Piano for 2.5 years-old children. 

The color is there to help our child to identify the key to press on the piano, without any immediate note-reading skill required, you can enjoy the fun of playing piano!!  I don't think it matters what color each key is being colored, cuz eventually when the child gets older, he/she will learn to identify the keys with the help of any color.  This is very cool way for really really young ones to enjoy the fun of piano playing.

Being a "music" teacher who have over 10 years of experience in teaching children under the age of 4, I must admit that the "traditional" way of learning piano may not be the best choice for children under the age of 5. 

Traditional piano lesson = one-on-one private lesson.  This will require the child (student) to be able to stay focus for the duration of the lesson, and have the ability to read.  In my many years of teaching youngsters, the reading ability can be obtained at the age of 4.  However, there is a big difference between being able to identify a note on paper than finding the correct key on the keyboard while reading the notes on the staff!  I believe this is where most parents are confused.  I my opinion, a child shouldn't sign up for private lesson until the age of 5 or 6 when one can be more independent and be able to understand things in a logical way.

And for those who want to start their child in piano lesson at a younger age, there are quite a few group methods available in HK.  Parson Music, Tom Lee Music, My Musikbox to name a few.

I hope you find this information useful