This is based on my experience living in Sierra Leone for close to a year, I can't speak for anywhere else in the world but I suspect this is typical. A few differences from existing answers:
- Clothes here are phenomenally clean every day of the week (not just Sundays or Fridays), school uniforms especially. People do have Sunday and Friday Best, but that's in terms of the clothes themselves, not the cleanliness which is consistent all week.
- People tend to hand-wash using cheap imported soaps (so it's not just some magic handmade soap that makes the difference). Don't underestimate how far into the bush certain imported goods reach, though the very remote villages make their own too. Simple things like leftover fat from cooking can work as a soap.
- The difference is visible indoors and outdoors, on light, bright and dark clothes, and against colourful backdrops, it's not just an optical illusion (though bright sunlight does really bring it out and make it show)
The difference seems to be hard work, hard scrubbing, and more attention paid to little habits that help keep clothes clean. Meticulous cleanliness of clothes seems to be a point of family pride, i.e. if you go out looking at all dirty, it reflects badly on your family, not just you. So they really do go above and beyond.
Since washing machines are both expensive and impractical, and unemployment is high, we hire a local housekeeper to clean our clothes. She hand-scrubs them meticulously, and they really do come back cleaner than from a washing machine. They also occasionally come back stretched a little... which shows how rigorous her scrubbing is.
It's the hard work that makes the difference, but there are also some other little habits I've noticed that contribute, for example:
- Putting things on the floor, even indoors, even temporarily, seems to be quite taboo here.
- Local people seem to be more careful about where they step. For example, they'll habitually walk in the road, out of the dust that builds up on the side of the road, trusting cars to weave around them, whereas I'll keep off the road even if that means walking through dust. Result: their shoes stay amazingly clean, mine get dusty very quickly.
- When carrying heavy items, I'll tend to carry them in my arms with the weight against my chest, whereas locals will tend to carry them on their heads, or if that's not possible, over a shoulder or just in their hands. I get dust, dirt and grime on my shirt, they don't.
- People seem to be a little more careful about wearing scruffy old clothes when doing dirty activities or using rags etc to keep clean with, then changing back. Similar to, say, my grandparents' generation.
Somehow, local people manage to keep their shoes clean even during the rainy season when half the country is mud. I've tried to work out how they manage that, but I haven't managed it yet.