After all that I ended up just doing this beautiful creation instead of trying to make a new gradient XD (it actually did take a lot of tuning and testing to get it targeting just the extreme hills biome without any others) It's a mix of too many other biomes using the bottom right so my experimental gradient changing the entire base color was too much (better to just target the extreme hills biome specifically), plus I decided to keep it consistently the same color at all altitudes, with the gradient it kinda ruined the effect with the duller color away from the corner used at lower altitudes