At the end of Iron Man 3 we're left thinking that the Iron Patriot armor, a recoloring/rebranding of the War Machine armor in the MCU, was the only one of Stark's armors left. It was left in the possession of Rhodes, who preferred War Machine over Iron Patriot.
By the events of Avengers: Age of Ultron, the Iron Patriot armor seems to have been retired, because Rhodes shows up to fight in the War Machine armor and at the end of the movie is still War Machine for his role in the New Avengers.
Websites and toy lines seem to indicated that both the Iron Patriot and War Machine armors, seen in Iron Man 3 and Age of Ultron respectively, are the War Machine MK II. I compared some images available of movie stills and toys, and there doesn't seem to be any changes. So, it seems for Ultron, War Machine just had the patriotic colors stripped.
The closest I've seen to an explanation is some articles claiming that they didn't need an "Iron Captain America" in Age of Ultron, so the design was changed back so there was only one red, white and blue character on the Avengers. This, however, was speculation, and I've not seen it backed up by any producer/writer/director statements.
Is there any concrete information given as to why the Iron Patriot design and branding was scrapped? Either an in-universe explanation from somewhere in the MCU franchise (webisodes or TV episodes included), or an out-of-universe answer by someone involved with the making of Age of Ultron.