The day Monster kind was sealed into Mt. Ebott, the mages added a little extra insurance. A curse.
Human SOULS may pass. But Human forms may not.
Young Frisk must navigate the dangers under the mountain, a little less human than before...
Yup! Grillby's flame temperature is based off of extreme emotions he experiences. So he'll be a soft yellow if he's happy and something like blue at the peak of anger.
...if he turns white then that will be a special level of rage and violence that few would survive.