Under normal illumination our rods are not playing much of a role in monochromatic vision -- our cones still are. Rods are indeed monochromatic, and as you say incredibly light sensitive -- but they don't kick in until luminance levels are very low, and they're also extremely low acuity compared with cones (because there are relatively few of them on the fovea, the area with highest acuity on the retina). But any given cone cell is still monochromatic -- it happens to be that some respond to blue, some to red, and some to green. So it's still largely cone vision that allows us to appreciate black and white photos.