I know how to create blocks and that there are several ways to modify them with jsons. However, I tried to produce a Green Fire block by replicating a common block and using only jsons.

Is this possible, or would I have to make a custom block?
(I'm trying to replicate just the texture, without the other attributes of the fire block)

Hi,

Create a new class that extends "Block" class and you need json for it in resources/assets/modid/blockstates directory and resources/assets/modid/models. You can generate json for it using a tool like misodes model generator. Here, atleast, are blocks explained at forge docs. 

Don't forge to look at vanilla code, like Magma Block is a good reference if you're trying a "green fire block". 

Modid should be replaced with your actual forge mod namespace!