Mars colony book I read in the 80's. The name?

Question

Read the book as part of my junior high LA class. Mars has been colonized, but there is one female that has been born on the colony. Everyone else is a transplant. She has to wear a mask, she is told, to protect her from various environmental factors that could harm her. She is allowed to take it off if she goes to a higher elevation where the air is safer. She also meets a boy that is new to the colony. He finds her and surprises her when she has her mask off. Her genetics have changed to adapt to the atmosphere of the planet. Her skin is green toned, it is thicker, almost scaley. Her eyes have changed. The boy looks like typical earthling. He is shocked by her appearance, falls down the mountain ( i think) and gets hurt pretty bad, or he might have died. It was not a series, just one novel.

Answer 1

It's not set on Mars, but otherwise this sounds exactly like Monica Hughes' Keeper of the Isis Light. It was published in 1980, so the time fits too. Olwen is the modified human who hides her appearance when she interacts with the colonists.

Quoting from the plot summary on Wikipedia:

Olwen is a young human woman living on the planet Isis as the keeper of the Light (a navigation beacon). Her parents having died, her only companion is a robotic DaCoP (Data Collector and Processor), called Guardian. On Olwen's 16th birthday (10th on Isis), the Guardian tells her that settlers are coming from Earth to Cascade Valley. Olwen is in distress thinking that these settlers will ruin her perfect world.

Guardian explains that she must wear a special protective suit to protect her from the viruses and bacteria the settlers might be carrying. One of the younger settlers, Mark London, falls in love with Olwen, and Olwen wishes Guardian to allow her to see Mark without her suit. Guardian refuses.

One day, Mark overhears Guardian discussing some of Olwen's blood samples with Dr. MacDonald and he thinks Olwen might be in trouble so he climbs up towards her house. When he sees Olwen, he suffers an accident and falls from the top of Lighthouse Mesa. This turns out to be because of his shock at Olwen's appearance under the suit.

Later, Guardian tells Olwen the truth about the death of her parents, and his subsequent care of her as her mother wished. To keep Olwen safe, he changed her genetically, so the ultraviolet rays from Isis' sun, Ra, would not harm her, allowing her to climb to Isis' mountain heights.

So this matches: Olwen was born on Isis; everyone else is a recent arrival; Olwen wears a mask when meeting the colonists; she can take the mask off when she's alone on the mountain; she becomes close to a boy, Mark; Mark finds her on the mountain when she has her mask off, is freaked out by her appearance and falls; Guardian has modified Olwen to better fit the planet.

The only thing that doesn't match is that it's not set on Mars.

The plot summary doesn't mention it, but Olwen's skin is indeed green:

Remembering Mark's freckles and the flushed peeling skin that she had noticed on the fairer settlers, she peered at her own skin, and then slipped out of her dressing gown so that she could see all of herself. Her body was strong and smooth, with no freckles or raw places or other deformities, but a nice bronzy green all over.

There are a few copies to borrow (registration required) at the Internet Archive, which is where I grabbed the quote.