minecraft 1.19.4 block/item_displays fire dimmer than actual fire

Question

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.

Which is when I noticed that the item_display was very obviously dimmer than an actual fire block. Even the block_display when using the default fire block looks worse. (Fire and Block_display Fire)

(Fire, 2 item_diplay fires, soul fire)

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.