You can pay extra mana for a spell like Radiant Flames only if an external effect is imposing an additional cost to cast it. That's a handy interaction with Vryn Wingmare and the like!
Welcome back! For today's Battle for Zendikar mechanic tip we'll be covering Converge! Converge works very similarly to the Sunburst mechanic from Fifth Dawn- the more colors of mana you sink into the spell, the stronger it gets. For Sunburst that meant charge counters and +1/+1 counters; for Converge, it just means more of everything! Normally you're "Guaranteed" a certain amount on Converge, since the spells have a colored component. For example, under most circumstances your should be a "minimum"