Is there a minimum requirement in terms of aperture for Junior Red Spot?
I think his father the Great Red Spot is a good object in even the smallest telescopes (e.g. 80 mm).
I don't know about minimum aperture, but the low contrast of Oval BA/Red Spot Jr. in recent years has favored good seeing since color contrast gets wiped out in poor seeing. In poor/average seeing only the darker brow around it has been visible in past years even in a large scope, while in good seeing folks describe color in as little as 60mm of aperture. As the seeing improves, so does the impression of color in my experience. This one has been very seeing sensitive for me, even more so than visibility of the white ovals, which are smaller (1/2 to 1/4 the diameter of Oval BA), but of substantially higher contrast.
There might also be an individual color vision sensitivity aspect at play. I find the color in this one very subtle in any aperture, some others do not. I don't doubt them, but I don't know how much to chalk it up to color vision differences, observing experience/technique, and/or seeing impact on color contrast.
Keep in mind that the GRS and Oval BA have varied in color greatly over the years. For a long time the GRS was a weak salmon color, rather unimpressive. In the past few years it has recovered its characteristic color. Meanwhile, in the past year Jupiter's equatorial zone has gone from creamy white with some bluer festoons, to a dark yellowish appearance.