If I have an Archmage Ascension in play with six quest counters and a Panglacial Wurm in my library, and I activate Chromatic Sphere's ability as part of the process of playing another spell, as in rule 601.2f, when do I attempt to start drawing the card? In particular, once the replacement ability takes effect and I instead search my library, can I start casting the Panglacial Wurm while I am still casting the other spell?
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Yes. It doesn't matter at all that you are casting another spell.
If the game instructs you to cast (or gives you to option to cast) a spell, you must (or can) cast the spell. (And that's the only time you can cast a spell.) Most of the time, the instruction is in the rules, but it's on a card in this case.
You might say "But I'm casting a creature when the stack isn't empty, and it might not even be my turn." Doesn't matter. Pangalcial Wurm doesn't say "Cast Panglacial Wurm if it's you main phase and the stack is empty" like priority effectively does.