Jupiter has an extremely thick atmosphere, and light from the sun penetrates only a short distance into it, which means that the majority of Jupiter looks like nothing but total blackness. Jupiter also receives very little light from the sun, making even the transparent layers of its atmosphere dim at midday.
An observer who descended into Jupiter's atmosphere from above would pass through many different layers before entering the zone of total darkness. At first, passage through Jupiter's atmosphere would be indistinguishable from passing through space. The Jovian atmosphere, like any planetary atmosphere, lacks a distinct boundary, so a falling observer only gradually would notice a build-up of rarefied hydrogen gas.
On entering the thick of Jupiter's atmosphere, the observer would be surrounded by swirling clouds of high-speed hydrogen and helium. The color of the sky would depend on local weather patterns and the latitude of the descent. Jupiter's upper atmosphere ranges from red to brown in a series of bands that girdle the planet from pole to pole and are discolored by storms that dredge up material from deeper layers of the atmosphere.
As the clouds closed over the descending observer, the light would dim and eventually disappear. Jupiter has no solid surface, and so the descent would continue into the planet's deep ocean of liquid metallic hydrogen. This is believed to be the source of Jupiter's immense gravitational field. Beneath that, Jupiter might have an Earth-sized world locked forever under unimaginable pressure.