If you intend to go for the All Clear achievement (beat Medium with every character), you can knock a good chunk of these out. Every time you lose and re-pick your character to continue (or any other character for that matter), you'll unlock another color/taunt.
I'd recommend Ish Agisaka's method if you don't intend to play with every character (like if you can't stand Balrog) or if you just need to snatch up the last few colors/taunts for a character you don't often use.
**Hypothetical numbers ahead - not an actual estimate**
The method works fine, but say it takes 3 min to start & quit almost 20 times per char. 3 min by 35 characters, that's over an hour spent getting the achievement "fast" but not really playing when it would just come naturally through playing.