because they reflect some light, so it doesn't keep going into the object until it gets absorbed. dirt scatters light with crystal facets. i don't know what the mechanisms are in poop and playdoh

but why is it brown instead of gray? the light that makes it back out is colored according to the dominant absorption mechanisms at play, and evidently in those substances they're better at absorbing blue light than other colors. i have some guesses about why that is but don't really know