The story goes like this: In January of this year, when MARVEL released the second trailer for 'Avengers: Age of Ultron', some European and American comic fans noticed that the Hulk's color in the trailer was a bit different. Especially, there was a significant difference compared to the Hulk's color appearing at other times in the trailer. Coupled with a promotional billboard circulating online, indicating that the renowned toy company Funko would release a Grey Hulk in its 'Avengers: Age of Ultron' toy series, this sparked a lot of heated discussion. Would different forms of Hulk have a chance to appear on the big screen?
▼ This scene at 0:49 in the trailer is the one that sparked heated discussions among many European and American comic fans.
▼ Other Hulks in the same trailer are clearly visible as green.
▼ 'Avengers: Age of Ultron' Second Trailer (Please pay attention to the 0:49 mark.)
▼ A promotional billboard circulating online, indicating Funko will release a Grey Hulk. (Its authenticity is currently unknown.)
So, what exactly is the "Grey Hulk"? We specially invited American comic expert POPO to introduce us to the various forms of Hulk that have appeared in MARVEL comics, giving all toy enthusiasts a chance to further understand the many facets of this incredibly powerful hero!
The creation of the Hulk was originally inspired by Dr. Jekyll and Mr. Hyde. Therefore, the portrayal of the character before and after transformation is actually the main theme of the Hulk's story, while the destruction caused by his immense power merely serves as a reflection for Bruce. In the mainstream continuity, it's been added that Banner suffered from multiple personality disorder due to childhood abuse. Thus, many versions of the Hulk each represent one of Banner's subconscious minds and deep-seated thoughts. 
First, let's explore Bruce Banner, the surface personality. The story has always portrayed him as a genius who suppresses his emotions, and whose high intelligence cannot be measured by known tests. Banner's transformation into the Hulk is usually involuntary, often triggered by emotions, especially anger and fear. Anger is also the emotion that people who suppress their feelings most want to vent. This is why anger is often mentioned in works related to the Hulk. 
Savage Hulk:
The Savage Hulk is what everyone commonly refers to as the "classic" Hulk, possessing limited intelligence and often referring to himself in the third person. Some scholars who study entertainment believe that the Savage Hulk represents the "ego" and "childhood" aspects of Banner's overall personality. Because he constantly reiterates to those who provoke and attack him that "he wants everyone to leave him alone and stay away from him," yet he is extremely grateful and loyal to his friends, his personality is actually quite simple. His physical characteristics include green skin and movement with ape-like leaping strides. 
Grey Hulk:
The Grey Hulk is the original version of the Hulk, the one who first appeared on the cover of Hulk's first comic issue. He later reappeared in stories under the name "Joe Fixit" working as a bodyguard. In fact, when the Hulk first appeared in 'The Incredible Hulk' #1, he was designed to be grey. However, due to the immature printing technology of the time, the grey often printed inconsistently, sometimes even appearing green. Therefore, starting with 'The Incredible Hulk' #2, the Hulk's appearance was changed to green.
▼ On the left is 'The Incredible Hulk' #1, on the right is #2 
Although this personality only possesses average intelligence, it does have knowledge unique to Banner (not IQ). His personality is lively, cunning, arrogant, manipulative, and cruel, yet he possesses a hidden conscience. Within Banner's overall personality, he represents the "adolescent" and "ego" aspects, essentially the rebellious phase.
The Grey Hulk is the smallest in stature among all Hulk versions and prefers to wear tailored suits. His strength is also the weakest of all versions; although he can increase his strength with anger, the rate of increase is much slower than other Hulks. However, the Grey Hulk uses cunning to gain an advantage, such as making people believe he is as unintelligent as the Savage Hulk, thereby causing enemies to underestimate him and allowing for surprise attacks. 
Professor Hulk:
Originally, Professor Hulk was conceived as a fusion of Banner, Savage Hulk, and Grey Hulk, hence his alias "Merged Hulk," possessing Banner's intelligence, Savage Hulk's strength, and Grey Hulk's cunning. Later stories revealed that he is actually a fourth personality, not a merger of the previous three, but possessing the distinct characteristics of each. Therefore, this personality represents Banner's ideal perfect image and is also known by fans as the strongest Hulk. In officially recognized crossover stories, Professor Hulk can defeat Superman. 
Mindless Hulk:
The demon "Nightmare," in search of a way to defeat his arch-nemesis Doctor Strange, began exploring the minds of his friends and relatives. He penetrated Banner's psyche, intensifying his fears and giving rise to the Mindless Hulk personality. Therefore, this Hulk can be said to be Banner's fear personality, symbolizing his inability to control the Hulk anymore. As a result, the Mindless Hulk is even more insane and irrational than the Savage Hulk; he wants to smash everything and everyone he sees, regardless of whether they are friends. 
Guilt Hulk:
The Guilt Hulk symbolizes Banner's regretful personality, forming early in Banner's mind. It initially appeared in his mind as the image of his abusive father from childhood, forcing him to relive the pain of his abuse and then reflect on the lives lost and destruction caused by the Hulk, attempting to take over his body when Banner's mind was vulnerable. Its characteristics include sharp claws, a body covered in spikes, and the ability to breathe fire. 
Devil Hulk:
The Devil Hulk symbolizes Bruce Banner's evil personality, representing Banner's resentment towards how the world has treated him, and is also a form of pure malice. Like the Guilt Hulk, it threatens to escape the confines of Banner's mind and destroy the world that constantly torments and abuses him, leaving nothing Banner cherishes. It has fought against other personalities within his mind. In later stories, the Devil Hulk's soul even merged with the soul of Banner's father, causing both mental and physical trauma to the Hulk. Its characteristics are a leaner build and a somewhat reptilian appearance. 
Green Scar:
The Green Scar is a personality that first appeared in "Planet Hulk" and "World War Hulk." He is smarter and stronger than previous personality versions, and even possesses leadership and command abilities.
Under the meditation training of his alien companions, the Green Scar Hulk's rage is most concentrated, allowing him to unleash maximum destructive power at a single point. This Hulk's endurance is also extremely high, as he can withstand the full power sonic scream of Black Bolt, King of the Inhumans, which the Savage Hulk could not. The Green Scar Hulk's characteristics include training in tactics and weapon usage, making him not only capable of frontline combat but also a strategist who can lead armies and plan tactics. Essentially, he is a personality that embodies the Hulk's vengeful nature and expresses Banner's leadership style. 
Doc Green:
Doc Green is a personality born when Banner attempted to cure his mind with the Extremis virus. Similar to the Professor personality, he possesses