But the majority is based on instinct.
"When an 'it girl' arrives, to an untrained eye, you wouldn't give her a second glance," Munro told CNN after Opening Ceremony debuted its spring collection at Fashion Week, where models were juxtaposed with flashy sports cars and Justin Bieber and Rihanna watched from the front row.
"Models are supposed to be aspirational, unreal characters," Munro said. "The fact that they are abnormally tall, perfectly proportioned and have amazing skin, it's because we're creating a picture. It entices someone to buy something."
Shelley said that when he and Munro make casting decisions, there are a limited number of diverse models. They have seen a growing number of Asian models, as that area of the world has become one of the fastest growing markets for luxury goods.
"All the shows essentially want the best of the best," Shelley said.
In a statement, the British Fashion Council said the lack of ethnic models is also a multitiered problem, from who the agencies decide to sign to what designers request.
"The British Fashion Council does not organize model castings for London Fashion Week, although, as its governing body, strongly asserts that all participating designers should recognize that London is one of the most multicultural cities in the world and should consider reflecting this demographic at their shows and presentations," it said in its response to the Diversity Coalition's memo.
This season, Munro and Shelley say that personality and quirk appeals because of the intensity of model turnover.
During the era of the supermodel, the faces of Cindy Crawford, Christy Turlington, Linda Evangelista and Naomi Campbell transcended the runway -- often into mainstream media -- for years.
Now, a model's look can be in and out of vogue in a matter of a few seasons.
It can be a matter of a model having the right look at the right time, whether heroin chic, like Kate Moss in the 1990s, or the androgynous look spurred by Agyness Deyn in the mid-2000s.
"The world has to want what that girl offers at that exact time," Shelley said.
But Shelley and Munro try not to cross the line from quirk into novelty.
"There's a fine line between an interesting event and a spectacle," Shelley said. "It's a spectacle when it becomes more about people wanting to identify with the strange."
Ross, the model with albinism, is a tricky one to cast, Munro said.
"That guy is unbelievably good-looking in an unbelievably bizarre way. But many designers haven't thought that way," he said.
Ross summed up his attitude with a hashtag he uses to his growing social media following, #InMySkinIWin, which he says promotes a level of comfort with yourself. He started it to raise albinism awareness, but has since expanded the meaning to just loving who you are.
"It's the DIY generation," Ross said. "Kids are becoming muses because they understand that what it takes to be successful is to be desirable and confident."
Follow Sarah LeTrent and on Twitter and Instagram for dispatches from New York Fashion Week.