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HOT and COLD are the two APPROACHES.  When you "defy the endless night" in Cold Fusion ie. perform a challenging action against risk, you must be acting in one of those two ways.  The basic dice pool is usually 4 or 5 dice (assuming an uninjured Pilot; fifth die from the potential for a Frame Quirk bonus), you always roll all the dice you have.  The different dice sizes are just a way to push probabilities toward one of the approaches: d6 is weighted toward Cold (66%), d10 toward Hot (60%), d8 is an even 50% split for both.

Bonus dice get added to the pool by Burning and/or Venting AFTER that initial roll which lets you deal with bad luck on your starting dice, have a chance against harder difficulties set by the GM, and/or to attempt actions your Pilot is less suited for.  Because "Failure Is Not An Option" you must keep adding dice to succeed for as long as you are alive, but the two consequences (HEATSEEKERS escalate the pressure, and SHIVERing) enter the picture if you can't manage to keep your majority of final dice results within the APPROACH you are rolling for.

Hot results are intentionally harder to get in order to 1.)  meet a specific theme, 2.) put pressure on the player to spend their Atomic resource, and ultimately 3.) push the asymmetric Action/Consequence curve toward the SHIVER result which drives the dramatic short-form death spiral story that this rpg is designed to tell.  It's like the final episode of an Evangelion-esque mecha show where spoilers: everyone dies.