The process of malting grains for brewing was known for centuries.
…the Greeks ascribed the invention of malt to the Egyptians. The art of malting, the key to successful brewing, is thus one of the most ancient of processes. The art found its way from Egypt to Tyre and Sidon and thence to Carthage, Greece, Rome, Germany, Gaul, the Scandinavian countries and to Britain.
The process has essentially remained unchanged in recorded history, with the exception of the advent of new equipment to make it more precise.
Malting consists of allowing grains to germinate to a certain point, followed by the application of heat to halt the growth and kill the sprouts. What remains is convertible starches, amino acids, and amylase enzymes which enable the mashing process – the production of fermentable sugars. We know that this process was well established by the time of Charlemagne by looking at the plans for construction of the St. Gall Monastery c. 820 AD. The monastery plans called for three separate breweries, a granary, a mill, and a kiln (presumably) for production of malted grains. This also establishes the earliest records we have of monasteries being involved in the production of brewed malt beverages.
Kilns used for drying the malt were primarily fired by wood, straw, or peat. In later centuries, anthracite coal may have been used as a cleaner, hotter source of both drying and roasting. Depending on the logistics of the kiln itself, the quality of the fire, and the proximity of the grains to the heat source, the individual grains might be lighter or darker. In earlier periods, grains would not have been separated by color, but would have been used collectively. This would result in a beer anywhere from a pale amber to a rich brown. Since beer styles were not defined to the level of detail they are today, color would have been largely dictated by individual kilns within each locale. However, by the mid to late 17th century, recognition of the differences between various roasts of malt was common. “…it then must be put on the Kiln to dry four, six, or twelve Hours according to the nature of the Malt, for the pale sort requires more leisure and less fire than the amber or brown sorts.”
“Colored malts resulted from uneven heat control which would have produced pale, amber and brown malts in the same batch, and likely in random distributions. Brown malt was also intentionally produced to reclaim slack malt.”
 Brookes, pg. 26
 Palmer, p.g 141-142
 Unger, pg. 27-29
 Fox, pg. 11
Brookes, Peter (2005). “Barley Breeding and Development in the UK, an Historical Perspective”. Brewery History 121, pp. 25-39.
Fox (1736). The London and Country Brewer, 2nd ed. Westminster Hall: Half-Moon and Seven Stars.
Hardy, Fred (1995). “Medieval English Brewing”. Retrieved 11 February 2015, from rec.crafts.brewing.
Palmer, John J. (2002). How to Brew, 2nd ed. California: Defenestrative Publishing Co.
Unger, Richard W. (2007). Beer in the Middle Ages and the Renaissance. University of Philadelphia: Pennsylvania Press.