Why are penguins black & white ?

Too much James Bond.

This type of coloration pattern is a camouflage called countershading. Many marine animals show a similar pattern: white ventrally so that it merges with the surrounding water when seen from below, and black dorsally so that they're more difficult to see from above, against the dark background.

It's seen in fish:

http://www.sharkspredatorprey.com/f_cntnt_ml_adapt...

http://lighthousemarine.net/images/fishpics/342%20...

Birds: penguins, auks, etc.

http://www.sennen-cove.com/images/little-auk.jpg

Mammals: killer whales, dolphins

http://students.ceid.upatras.gr/~pirli/whales/orca...

and many other aquatic animals.

To blend into their environment better. They are white on their tummies so predators below will have difficulty seeing them. They also flock and the mass of black and white will confuse predators making it difficult to pick one out of the group.

cos if they were green & pink they would look silly

because they are not blue