The color of the saguaro needles changes over time. The newer they are, the whiter they are. Over time, as in years and decades, they change to black. So, the tops of the saguaro trunk and arms are covered in light-colored needles, and bottoms are covered in darker needles. This image, therefore, is of a young saguaro as I was able to photograph white needles along the main trunk while standing next to it. For older saguaros, these spines would have been black, and this would have been a very different picture.