I've been starting to like reds a little bit more than I use to but still don't like them as much as blues :/
Here's the way I usually put it. Let's say we ranked how good each attack is for each color. For most of the things that reds would be a 5/5 in, blues would most of the time be a 4/5 at worst. Anything a red is really good for, blues are really good for as well, as they deal almost the exact same damage and reds have slightly higher knockback. Reds however do have higher priority which I'm going to look into. However, the only difference is that for things that blues would be a 5/5 in, most of the time reds would be like... a 2/5 or a 3/5. Reds have the problem of being mediocre for grabs while blues excel in grabs (if nothing else you can get 12% from a blue dthrow!). The way I look at it, Reds are good at most things but below average at grabs, while blues are amazing at grabs and either great or above average at everything else. So it's not that reds are necessarily bad, just that they're outshined by blues.
Wow that was a lot longer than I expected it to be XD But yeah, whites are most likely the worst, however they have their moments and their matchups where they can shine.
As for the mk matchup, I would say yellows and purples. I'm going to test a few things to see if reds do any better at beating out any of his attacks than the other pikmin, but the range of yellow pikmin work wonders against him. Yellow fsmash in particular, for me, as the hitbox is biggger as the pikmin falls so it throws off the mk's spacing when trying to punish it. However, I find that having too many yellows can be a problem, as it gets the opponent use to the range difference, instead of it being a surprise that throws them off. Purples' use is mainly that purple pikmin toss is amazing, throws up mk's approach and pressure game and landing early kills is extremely important.
Keep the questions coming Jupz :D
Oh also, I plan to start updating this thread soon, it needs to be. It might end up looking a tad different afterwards, we'll see.