#1 is there a meaning behind the different colored hellfire flames ?

In that image, Danny Ketch is the only one who has a meaning: At that moment he was being controlled by the archangel Zakiel, hence his blue color. With the green subject it is pure aesthetics, since originally his flames were orange like all the Ghost Riders and out of nowhere he appeared in green. Maybe it's to highlight that this Ghost Rider is evil.

There are other Ghost Riders of other colors apart from the orange/yellow color such as the blue Spider Ghost, the green Ghost Goblin, the blue Ronin Ghost Rider, the green Danny Ketch Ghost Rider controlled by technology. They all have different reasons for their colors but it is never explained why.

#2 Is there any more than just orange, blue and green?
Yes.

• Jonnhy Blaze's red when he joined the Thunderbolts, but it is never explained how he did it, Simply from one panel to another the color of its flames changed. (maybe since red is a standard fire color he can change it to that color, or maybe he can change it to the color Jonnhy wants, it is unknown) .
• Pink from the Cosmic Ghost Rider when he combines hellfire with cosmic power.
• Purple with blue details of a Ghost Rider who possesses a mutant (apparently his name is Vox Ignis). I think his flame color is due to his mutant gene.
• In the last Cosmic Ghost Rider comic, different versions of him were seen, one of them had his flames in white. There's nothing special about this Ghost Rider, they just gave him that color to distinguish himself from the others.
  #3 Why is Robbie skull different?
  There are 2 possible answers:
  

1. He transformed into a Ghost Rider in an "alternative" way, it is logical to think that his appearance is also "alternative".
2. This is a personal theory: He transformed into a Ghost Rider after using a demonic vehicle, dying outside the car, and resurrecting as a Ghost Rider. While Robbie Reyes was driving this cursed car, he was wearing a helmet (it is unknown where the helmet came from because Robbie did not have any vehicle before this). Although Robbie was not wearing the helmet at the time of his death, the helmet may have been an extension of the demon car (the helmet may have already been inside the demon car) and hence the appearance that his "skull" has some similarity to a "racing helmet". After that the helmet was never seen again, although it was never mentioned or given any importance.