That’s an idea you see early in this topic, with caveats mentioned, before ChatGPT did start offering a method for transparency again.
On API where OpenAI still hasn’t given transparency free users can request on ChatGPT, if you want to do green-screen chromakey, I offer that or other instructed then sampled colors you can preview (for operations on loaded or pasted local files also.)
Green or magenta or other strong edge-case color is OK if that is a color that will be held-out in the imagery, but a white or black matching the destination background also can perform well if the AI, by instruction, will keep those strong signals out of the main image. “Halo expansion” control will consume more color adjacency to a detected transparent to also make bleeding key color transparent.