Definitely not on purpose - that makes no sense at all from a business perspective. It's suggested every year when the most popular color is limited, but it just a case of actual supply/demand.Yes only because Apple does this purposely to create "hype" for their newest iPhone. They do this with every new color that's introduced to their flagship phone. Pure marketing.
However IMO Apple dropped the ball this time around because they could have put the squeeze on Samsung and their failed Note 7 release/recall by having more quantities and colors of the 7 and 7+ available on release day - potentially allowing those customers to jump ship.
No way they could have known about the impending recall of Samsung phones. They have been making as many as possible since at least August.
EDIT:
https://www.macrumors.com/2016/09/05/apple-orders-iphone-7-parts-samsung-recall/