Or the manequin holding up that shirt in the sunlight doesn't have as much heat to give off as the others do because the lighter shirt reflected more heat away prior to that.
I'm not aware of any heat-trapping properties of the colour white, other than it'll reflect visible light. Though I think how it handles IR and UV light might be more relevant than visible. Like would a black shirt that reflects UV light well feel cooler than a white shirt that doesn't?