Is it true?

I asked my friend if it’s good that I saw some dark poop. She said “Yeah, the darker the better.” My bunny’s diet consists of 1/4 cup of pellets, 1/2 cup of veggies such as celery and kale, unlimited hay, and unlimited water. Is this true? The lighting makes the poop look a bit lighter

That just looks like normal, healthy poop to me.

Darker poo isn't necessarily better. I think the richer the diet the darker the poo. If they eat a lot of hay the poo will be a little bit lighter but that is ok because it means they are getting a lot of fiber. The poo in the photo looks healthy to me.

Also, the poo tends to get lighter in color the longer it sits. So that which is "freshly pooped" will tend to be darker.

Darker poop that isn't necessarily smaller in size(small dark fecal droppings usually happen with the beginnings of stasis or dehydration), usually is from more protein in the diet such as when fed alfalfa, very leafy soft grass hay, or fresh leafy grass. The more indigestible fiber in the diet such as from stalky stems in hay or ingested wood pulp from branches, will cause the poop to be a lighter brown color and usually bigger in size.

I agree with what was said already if you feed more vegs poo will be darker and wetter one of my rabbits eats lots of hay maybe twice more than others so her poo is usually very light and greenish but when I give her vegetables poo is darker, almost black. Dried poo is much lighter I also agree with that. Your photo shows normal healthy poo, well done.

Besides, my lop girls are bigger and their poos are larger too, I've two males one lionhead other dwarf/lionhead, their poo is much smaller and drier.

I feed 95% forage in summer, the poo is almost black. That changes in winter when most of their diet is hay.