As I saw in one of the postings here, field spells and also explosive potions only harm those that are red, grey or orange to the caster.
Can you please tell me how to modify it so everyone (even the caster) can be harmed by them?
Doesn't appear to be.
You cannot hurt anyone who isn't red, orange or grey to your character with AoE spells and explosive potions fall into that category.
The feature was added some time ago, I've read it right here on this forum.