If (PRODUCT)RED does stick around on the iPhones, I think it is almost certain it will be exclusive to the latest model - so a year from now, there won't be a RED iPhone 7 in production, instead it will be the 7s or 8 or whatever in my opinion.
However, we might not see a RED model during the launch quarter at all. Apple is under such intense pressure in terms of year-on-year iPhone profit and revenue comparisons and the donation element will be significant in a launch quarter on a popular colour. Even if the donation is just 5%, that will be a significant downward pressure on the figures when Cook's job is to report record revenue and profit.
Don't be surprised if the RED model becomes a yearly mid-cycle release.