Book 1 should use BLUE colours as it revolves around water (book 2 would use white as it revolves around snow and book 3 would use grey as it takes place in the city+concrete building).
David Archer, the protagonist, is a twelve-year old BOY, brown hair, brown eyes.
He has a DOG, a Jack Russell Terrier, brown and white. The dog plays an important role in book 1 and 2, but NOT in book 3.
David acquieres an alien skill which allows him to communicate with animals telepathically. In book 1 the main animal at the end is a humpback WHALE (book 2 has a grizzly bear, book 3 has a bee).
The ALIEN survivor is handsome, tall, has dark, honey-coloured eyes, high cheekbones, a square chin and a straight nose, olive-tanned face and white hair. When he crashes to Earth he is wearing a black outfit made of one piece. Then to blend in with humans he wears jeans, brown jacket and a fur hat with ear flaps to hide his white hair. He can manipulate WATER.
The alien ENEMY is tall, has spikey, white hair and honey-coloured eyes. He can shape-shift to take on the form of other human beings. He changes into a handsome police officer with a bald head and green eyes who wears a neat suit and tie (think Men In Black).
Please check out my previous book cover attached, which had a different title and pen name ("The Chilliwack Fall" by A.J. Block). Although I loved this book cover, readers were confused because they thought the book was about a humpback whale. But it is mainly about a boy and an alien who become friends as they flee from the alien enemy and government agents.
In this previous book cover, the element of a FALLING STAR in the background could be used in all 3 books. This is a good depiction of the alien voyager falling from the sky. This falling light could be used in book 2 (sun shining behind a mountain) and 3 (the alien departs) to show the sci-fi/space element of the story.
Lastly, the family-friendly ROMANCE is between the alien voyager and David's mother: ash blonde hair, shoulder-length, green eyes. But this is a book for children/teens and romance is not the main theme.