It appears the J-20 has a different style of camouflage on the upper surface than the lower one. The upper surface has jagged, sharp edged patterns, the lower a soft mottled pattern.
My guess is that they may be intended to emulate cloud patterns... You will see its belly with the sky and clouds as the back drop when you look up (as shown in the photo). So they want the belly camo pattern to emulate sky and cloud.
When you are above the J-20 and look down, you will see the J-20's top with ground as the background. So they have a pattern that emulates the jagged ground features (farm land, mountain patches, etc).
This is the same as fish in the ocean. Most fish have dark back (to blend in with the dark ocean bottom when you look down on them) and light belly (to blend in with the bright sky at the ocean surface when you look up). Natural camo...