Who said anything about customer demands? Reading comprehension called, it's looking for you
I'll ignore the idiotic "reading comprehension" blabber and quote your original "claim":
This is a marketing strategy. When people see my iPhone 6+ they immediately ask me if it's iPhone 6s+, cos from outside there is no difference.
Space grey is the top selling color. If there won't be any design change on iPhone, iPhone 7 will NOT sell as many as 6 and 6s. The only way people can show off their "new" iphone would be if it was a different color than 6/6s.
Apple is targeting space grey by making it an obsolete color, to push more people into "upgrading" to "dark blue". If everyone has seen the reports, 50% of iPhone buyers don't upgrade their phones for 2+ years.
edit: Oh, and I won't be surprised if Apple introduces a new iPhone color every year making the newest iPhone more distinguishable on sight
According to your "theory", people want to show off their new phones and the only way they can do that is by having a phone with a new colour. Even you should by now understand that your "claim", as quoted, implies that there IS a demand for the new colour. And Apple is feeding that demand by introducing a new colour.
What is your problem, if I may ask