The Astounding Broccoli Boy, by Frank Cottrell Boyce is a caper across London (with interludes in a hospital isolation ward) featuring two green boys, joined later by a green girl and a penguin, with a cameo appearance of two other penguins who aren't important.
The fact that the three kids are green is, however, important--it is the whole foundation of the plot. Rory, the main character, starts off brown (his dad's from Guyana) and turns green--really truly green, for no immediately obvious reason. And so he's quickly carted off to an isolation ward in the local hospital, where to his great horror he finds another green kid already in residence--Tommy-Lee, the bully who'd been making his life miserable for months.
But turning green has (perhaps) given them superpowers-Rory is sure that his brain now works at 200% capacity, and that he can teleport (slightly and instinctively). Tommy-Lee can open doors locked by coded keypads in his sleep. So the two kids join forces to make their green-ness part of their new super-hero identities. Good turns out to be kind of flexible and hard to pin down--is giving zoo animals freedom good? Is taking peoples money in exchange for posing for pictures with them good? Is (unintentionally) convincing the city of London that there are aliens taking over the city (the obvious explanation for the little green men who are Rory and Tommy-Lee) good? Is smuggling a third green kid, a girl this time, back into the hospital really what those in authority want? Breaking into Buckingham Palace on a "borrowed" milk van? Not so much. London, already on edge because of a mysterious pandemic (the Killer Kitten virus), doesn't exactly welcome Rory and Tommy-Lee adding to the confusion.....
It's left clear (ish) that the kids don't actually have superpowers, but they definitely did turn green. Which makes it speculative fiction, because people don't turn green in real life (much).
And it's fun in its own way, once the rather tiresome business of wimpy kid being bullied by big bad kid is gotten over with (I am tired of bullies becoming best buddies). But if you like books that are mostly bouncing between humorous romps, you'll enjoy it--there's plenty to chuckle at. It doesn't have much that goes any deeper, though, and so I myself found it a tad disappointing.