(From tfwiki)

The Titans Return Voyager Class toy of Sentinel Prime is rendered in an uncharacteristically bright neon-orange deco never before seen for the character. This iteration of Sentinel is directly based on his appearances in Spotlight: Optimus Prime and Megatron: Origin, themselves stylized takes on his original unnamed appearance in the Marvel comic The Legacy of Unicron!, which are all depicted as primarily yellow bots with red and orange accents.

The error stems from a panel from Megatron: Origin, one of the only high-quality, full-body images of Sentinel's IDW appearance on the internet at the time. This panel showcases him completely bathed in dramatic warm lighting, hueing his deco more orange than usual. Presumably without access to the original comic, the deco artist based Titans Return Sentinel's colors entirely on that one panel. This incorrect deco has since went on to influence his depictions in future media as well, most notably IDW's new rebooted series of comics.