For field work it is very common to use the Munsell soil color charts.
http://www.fao.org/3/a-a0541e.pdf
https://www.amazon.com/X-Rite-Munsell-Soil-Color-M50215B/dp/B007E9JERA
Munsell Soil Chart is good steel method, but in a digital era, I programmed some colours software analyser than can improve for soil textures. This small software can determine in RGB colour system in the 8-bit system exactly 16,777,216 different colours.
we use the 430 nm reading of a 4.43m HNO3 1:10 extract. You can calibrate this readings with common methods of OM analysis.
Munsell Soil Color Chart is the standard procedure being followed. For other methods, calibration for native conditions is essential for meaning and use observations / collection of information.
Interesting discussion . I agree with Dr Ghafoor , most common of all is the use Munsell Color Chart , unless calibrated with other methods using localised conditions. Reducing soil conditions as indicated by: gleyed soils (blueish or greenish gray) immediately below A horizon, matrix chroma (predominant color) less than or equal to 1 (using Munsell Soil Color Book) ; bright mottles immediately below A horizon or 10 inches and/or matrix chroma of less than or equal to 2 (if soil has mottles; color determined with wet or moist soils) or Iron (reddish brown) and/or manganese (black) concretions..However , such color determination doesnot hold good in coarse textured soils , because of reasons h igh organic matter content in the surface horizon. The mineral surface layer will appear darker than the mineral layer below it. Dark vertical streaking of subsurface horizons by organic matter movement as water table fluctuates. While , in wet Spodosols , accumulation of organic matter 30 to 60 icm below the mineral surface....
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