I always wonder how chemists make compounds that are colorless. For example, most shaving cream contains lots of chemical components, yet is completely white. If I would mix even a small number of random chemical components the result would certainly not be white (probably closer to brown). Perhaps someone can explain how this works from an engineering perspective.
(And also interesting in this context are odorless compounds).
Shaving cream is mostly soap, isn't it? Also I wonder that it's whiteness (reflects all colours equally) isn't at least partly due to it's physical structure (lots of bubbles).
However it is true that randomly mixing lots of different coloured components together does mostly end up with something brown (cf playdo and shit).
when talking of objects rather than lights, "color" is reflectance or transmittance that differs significantly between the response curves of different kinds of cone cells in your eye. those response curves are fairly broad, overlapping greatly and each having significant response throughout the visible wavelength range
as i understand it, pure substances in liquid or gaseous form mostly have very narrow absorption spectral absorption lines in visible and near infrared, so they can only remove a tiny fraction of the energy from a continuous-spectrum light source; things like nitrogen dioxide, chlorine, and transition metal ions which can absorb light over a wide spectral range are unusual. (the uranium ions we're talking about here are transition metal ions.) in solid form the absorption lines do widen, but still usually not enough to appear as "color"
sometimes a wide absorption peak is so wide that it absorbs light across the whole visible spectrum and the material looks black
so the vast majority of pure substances are colorless. if you mix a small number of random chemical components, most of the time, the result will be colorless. if it's not black, whether it appears clear or white depends on whether it scatters light; interfaces between birefringent crystals or materials of different refractive indices scatter light. in canned shaving cream, what scatters the light is the interface between the liquid solution and the bubbles of propellant. it actually takes a lot of dye before such a fine foam stops looking white because the bubbles are so good at scattering light
so, if you mix a small number of random pure substances, the result probably will be clear or white. but most substances you encounter on a day-to-day basis, such as dirt, tomato sauce, paper, poop, or concrete, are not pure; they're inhomogeneous mixtures of literally thousands of different pure substances. such a mixture can easily absorb light over a wide spectral range even if each of the substances only has narrow spectral absorption lines. and usually a few of the substances contain transition metal ions and individually absorb over a wider range of wavelengths
i hope this is helpful
as for odorless compounds, most of them are just so far from their boiling point that not enough of them evaporates into the air for your nose to detect their vapor. there aren't very many odorless volatile compounds
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
(And also interesting in this context are odorless compounds).