2020-08-07
Once Pentad Prism has run out of charge counters, it remains on the battlefield.
2020-08-07
Sunburst checks what mana was actually spent to cast the spell. If an effect allows you to spend mana "as though it were mana" of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.