I have planed and given a coat of varnish to some rough bow blanks and then photographed them to show this variation. It’s better to see the actual wood but at least the photos give an idea. The first photo shows two high quality sticks from Espirito Santo state in Brazil showing the variation in color ranging from yellow to brown. The second photo shows an outstanding piece of Espirito Santo wood below a very poor grade of pernambuco wood that is too light and weak for a quality bow.
The issue of wood color is further complicated by the fact that pernambuco wood has often been treated to color the surface a dark reddish brown. Among other things, the natural dye in the wood can be set by applying nitric acid, by exposing the stick to ammonia fumes or sometimes both. This was a standard practice from the mid-nineteenth to mid twentieth century and bows were uniformly treated this way whether of high or low quality. For example, bows by fine bowmakers like Lamy, Vigneron and Sartory were treated with nitric acid. The effect was only on the surface and apparently any harm it did to the wood was insignificant. James Tubbs used a process that turned his sticks nearly black.
Here is a close-up picture of two nice bows by Alfred Lamy, a violin and an octagonal viola bow dating to around the turn of the century. These bows were treated with nitric acid and the wood was probably naturally lighter in color.
During the last three decades the new generation of bow makers has usually chosen to make bows without darkening the wood. This is in part because these younger makers were willing to invest in wood of exceptional beauty and this encouraged dealers and millers of the wood in Brazil to locate the best qualities. It is simply unnecessary to treat beautiful pernambuco and doing so risks losing some of the wood’s transparency and purity of color. But sunlight brings up the color in a nice way as the ultraviolet rays oxidize the surface. So we often put our bows in the sun for a few hours or even put them under an ultraviolet lamp in the winter.