The Astounding Broccoli Boy by Frank Cottrell Boyce

Rory Rooney likes to be prepared for all eventualities. His favourite book is 
Don't Be Scared, Be Prepared
, and he has memorized every page of it. He could even survive a hippo attack. He knows that just because something is unlikely doesn't mean it won't ever happen . . .

But Rory 
isn't 
prepared when he suddenly and inexplicably turns green.

Stuck in an isolation ward in a hospital far from home with two other remarkably green children, Rory's as confused by his new condition as the medics seem to be.

But what if it's not in their genes, or a virus, or something they ate? What if turning green actually means you've turned into a superhero?

Rory can't wait to make it past hospital security and discover exactly what his superpower might be . . ."