Sunburst is a fairly unique ability. As a permanent with Sunburst enters the battlefield, Sunburst checks to see what mana was paid when casting the spell, regardless of what the cost was for.

So if you want to cast Engineered Explosives when your opponent controls Thalia, Guardian of Thraben, you can choose any value for X, pay 1 additional for Thalia’s ability, and it will enter with counters based on what colors of mana you paid for the total cost. For example, you can choose X=0, add the 1 for Thalia, and pay U for the total cost. The Explosives will enter with one counter. If you wanted to cast the Explosives with 0 counters, you’ll need to have colorless mana, or whatever you pay for Thalia’s 1 will cause it to enter with a counter anyway (meaning you’re probably not going to be able to end up with Explosives with zero counters most of the time).

Similarly, with Trinisphere, you must pay at least 3 mana for any spell. Again, you can choose X=0, 1, 2 or 3, but you’ll still have to pay 3 mana of any type. If any of that mana is colored, Sunburst will know about it. To cast Explosives with zero counters in this case, you’ll need 3 colorless mana. For 3 counters, you’ll need 3 different colors of mana, and so on.

Today’s Rules Tip written by
Josh Stansfield, L2 judge from Orange, CA