Blue
Ravenous Bugblatter Beast of Traal
If the DM throws an adult red dragon without foreshadowing you don't want to go that way, forces it to be a combat encounter, and provides no course to retreat, then it's on them.I know what is unbeatable, and if a DM throws an adult Red Dragon at my 1st level character, that's on them, not me, and is exactly why I wouldn't try again. There's no challenge there, nothing.
You ignore the information that there's a red dragon up in the mountain, enter it's lair, attack it, and expect that it will be combat balanced so that you have a reasonable chance to defeat it - nothing wrong with what the DM is doing there. And even then the DM could still save you from yourself by providing escape - perhaps passages too small for it to follow, or a goal that it would want you to do for it in exchange for not dying.