Umm, are you sure
about that? Here's the page of screen captures from the epi' in question, and the balloon decoy was a pale blue.
And I don't think this was one of the "touch-ups" just for the DVD release as I remember it being blue in the original mid 70s airing and the reruns on Nick' in the 80s.
Yeah, a lot of things were painted pink like the tribbles and the Kzinti spacesuits, but the decoy was not one of them.