Awakened Zoan Devil Fruits might be the strongest ones in One Piece, but due to their mysterious reason, it is hard to pinpoint the full extent of their abilities. However, one key hint of a Zoan fruit's awakening is the appearance of a cloud around the user. Moreover, the color of this cloud might be the true representation of the user's control over their Devil Fruit.
The colors of the clouds of the Gorosei and Lucci (black) signify that they dominated the will of their Devil Fruits, whereas Luffy's color (white) signifies a proper symphony between the user and the Devil Fruit. This theory is further strengthened through the control of these three in their awakened Devil Fruit forms and how an antagonist has a cloud similar to that of Luffy.
Disclaimer: This article contains potential spoilers from the One Piece manga and has the author's opinion.
One Piece: Exploring the colors of clouds that appear during the awakened state of Zoan Devil Fruits

Zoan Devil Fruits are one of the three main types of Devil Fruits that give their users the physical capabilities of an animal. The models of these fruits range from normal ones (Dalton's Ox-Ox Devil Fruit) to mythical ones (Kaido's Azure Dragon Devil Fruit). However, unlike other classes of Devil Fruits, this type of fruit has a will of its own even before consumption.
So, the powers of this Devil Fruit exceed its normal ones whenever awakened. However, only a few characters have awakened their Zoan Devil Fruits. Some of these examples include the Jailer Beasts from Impel Down, Lucci, and Monkey D Luffy. However, having an awakened Devil Fruit doesn't mean a user would be overpowered as they might lose control of the Devil Fruit's will.

For instance, as reported by Eiichiro Oda, the Jailer Beasts of Impel Down had no control over their bodies as their Devil Fruits' wills took control. On the other hand, both Luffy and Lucci had power over their control (to some extent), but what differentiated them was the color of their awakening clouds. The color of these clouds might be the deciding factor over a Devil Fruit's control.
Luffy's white cloud symbolizes the perfect division of control between the protagonist and his Nika Devil Fruit. On the other hand, Lucci's black cloud symbolizes the antagonist's dominance over his Devil Fruit's will. According to this criteria, some other presumably awakened Devil Fruits might also make sense.

The Gorosei's awakened Devil Fruits (which are also considered yokai forms, instead of Devil Fruits) also have a black cloud around their necks, which would fit their evil nature. This means that they dominated their Devil Fruits' wills and used their powers for evil.
However, Rimoshifu Killingham, as teased through One Piece chapter 1142 raw scans, has a white cloud around him, meaning that he might not be in full control over his awakened Devil Fruit powers. It might be hard to speculate as to which control (partial or full) makes a user stronger. However, awakened Zoan Devil Fruit users could be classified based on their cloud colors.
Analysis and final thoughts

A common misconception before was how the color signified the true nature of a user (black representing antagonists and white representing protagonists). However, the appearance of Saint Rimoshifu Killingham might have changed the entire thing. Nonetheless, the full classification of this Devil Fruit type might still be a big mystery.
Related links
- One Piece chapter 1142 spoilers
- Gunko's actions in the latest One Piece spoilers all but confirm Oda's Germanic inspiration
- All known Holy Knights in One Piece, explained