As a retired structural materials engineer and now a professional photographer, I have personally handled and worked with soil/rock/concrete/concrete masonry units-refered to as concrete block - bituminous concrete (also known as blacktop or asphalt paving) - for more than 40 years.
Rock comes in all shades of the color spectrum depending on the chemical makup of the rock. The most comon colors for dolomite/limestone are from a blueish-gray to light gray to dark gray. Sandstone colors vary from light yellow to tan to reddish tan to shades of brown. Shalestone varies from shades of brown to shades of red/brown. Gravel can be any combination of the afore mentioned colors.
Concrete is a mixture of portland cement - normally gray, but can be tan, and also comes in white. (Adding a colorant such as iron oxides - any color from yellow to red to black) changes the color of the paste - portland cement/water.) Getting back to Concrete - a mixture of portland cement - sand - crushed rock or gravel - water. The proportions vary depending on the application, but usually consists of 1 part portland cement, 2 to 3 parts sand, and 2 to 4 parts crushed rock or gravel. The resultant cured concrete takes on the shade of the sand/cement paste that holds together the crushed rock. It can varies from light gray to tan to reddish - again the color of the sand imparts the hue.
Bituminous Concrete - a mixture of black asphalt - sand - crushed rock or gravel. The initial color of a paved road is black. As traffic wears off the surface, the color of the road changes to the color of the exposed rock and sand that is held together by the black asphalt.
Concrete masonry units - block - is the color of the concrete mix and usually assumes the hue of the sand/cement mixture used to make the units. The normal color is gray, but can vary from dark gray to tan.
I hope I have not bored you too much - not my intent here. I just wanted to make you aware of the wide range of color hues you may encounter when looking at what we consider normal building materials.
My suggestion - unless you have seen the material prior to photographing it - don't assume a neutral hue/color.
"reprints" of selected articles from Usenet and other forums
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The following information was posted by Mike Demyan, on the http://tech.groups.yahoo.com/group/colortheory/ mailing list. For setting neutral points, there is some good information here:
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