Well, I have 2 white/grays and one black. They are both hard to keep clean but for different reasons. The grays get stains and yellowtails while the black one takes forever to get dirt and dust off. A quick wash usually takes care of the grays.
I would also say chestnut is the easiest. I owned one of those. I've also owned several grays and thought they were the worst.......until I got a black. Even when she looks clean, if you brush her it brings up all the dust. I would say black is the worst! Grays can turn yellowish but the dust doesn't show as bad.
They all get dirty. My palomino paint is actually surprisingly easy to keep clean. Unless it's an obvious grass or poo stain any yellowing is hidden by his blond/darker blond coat. I don't have to worry about his tail being a little yellowed because it kind of matches. My sorrel mare with a dark mane and tail is always dusty looking.
I don't know if I have a favorite color horse, I love things about them all. Palomino paint is such a unique looking color but sometimes the colors come out muted and dirty looking. I think how appealing a color is also depends on confirmation. A sleek, black Arabian would look better than a pinto horse of the exact same build IMO. And a spunky little pony should always be piebald. I think drafts with feathers look better in darker colors too (even gypsies look best with heavy pied markings).
I just got an Appaloosa so I will update when I know how she does! I had a white paso before and it depended where he lived. I had him at two places and one he was pretty clean. The other he was always orange from the mud!
Bays are always a good choice. They're easiest to keep clean, the coat color hides mud well, and they're pretty much always cute. I don't think I've ever met an ugly bay . My favorite horse color has to be blue roan, though. They're just so cute! My horse is chestnut with a flaxen mane
Also, I leased a POA for around 3 years. Bay marbleized roan with SO much white, he honestly looked more leopard. An absolute NIGHTMARE to keep clean!!! I would have him all clean and lovely the night before a show, and I would arrive at the fairgrounds the next morning to discover he managed to lay in his poop and STILL get stains under all his pajamas! He also had varnish marks on his legs, so there were countless times when judges mistook them for mud/dirt/pee stains and took points off
As @trailhorserider said, a mud colored horse looks the best when dirty. I had a mud colored appaloosa that almost always looked nice.
On the other hand I had a white appaloosa with black polka dots who kept herself super clean all the time. She hated mud, water, and being dirty. My neighbor called her my "appaloosa American princess". She only rolled where it was clean. The downside to a horse like that is that she HATED crossing muddy places or water. She was very prissy.
I KNEW I was grooming the darker ones wrong... it drove me batty with the dust. I am really thorough and it wasn't until I watched the manager of the place do a less thorough job and then a wipe down with a baby wipe for the dusty parts. Absolutely mind breaking. I once curried a jet "black" type so much that I got reprimanded and he had to be hosed down lol!
I don't think its about colour. I think its about the HAIR and stabling! A short hair without feathers? Excellent. With feathers or thick coat? Oh no. I spose you can clip...
I also have recently transitioned to full 24/7 turnout and can say it is 10x easier to keep my grey mare clean and she hasn't got hairy-hairy legs either. No more urine or poo stains to remove. Just rinse off the mud and bam good to go. And if I can't wash wait for it to dry and curry and she comes out looking great. Downside IS the tail but at least its one of the easiest things to clean and reasonably so in colder weather (with a tail guard and towel to dry while being pampered).
My favorite color to look at is grulla (blue dun). My favorite color is the color of whoever my favorite horse is. Currently, solid brown.
I don't care for white on animals. The more white on them, the less I like it. I was traumatized as a child getting our tobiano pony ready for 4-H shows -- he had a black head and one black spot on his back, and the rest of him was white. I also don't like pale or washed-out-looking colors like cremello. I love dark dappled grays but not what they turn into, so much.
I was just thinking about the last time I gave my horse a real bath as opposed to rinsing the sweat off after a ride. It was more than two years ago, back when we made the mistake of burning brush piles in the pasture. The horses loved to roll in the ashes. And then it would rain ... ugh!