In the latest datapack I'm working on, I wanted to play with different fire colours, namely a green copper fire and a bright yellow blaze fire. I was happy with the recoloured textures I'd made, and then copied and modified the existing fire block models into 1 model I could create using the new item_display, based on a fire charge CustomModelData.
I've tried putting a 15 level light block inside the fake fire, but while it lights up the area, the texture remains darkened. I vaguely remember that there is a way to change the brightness within a texture file, but I cannot find that option anywhere, and Googling the issue only comes up with resourcepacks for brightening the game.
At this point, I don't know if I need to change the root item, or if there's some hidden option I need to write into the texture JSON file to made it brighter.