I came to this stuff with a background in photography, and getting a greyscale "right" can be tricky. Photographs were routinely adjusted, to correct for the differences between the eye and film, and to show colour differences that were lost. All this is for black-and-white film: The film is more sensitive to blue light, and older films didn't respond to red light (which meant makeup for cinema actors was a bit odd-looking). Photographers routinely used a yellow filter: yellow because it blocks blue light. And green trees reflect red and infra-red light. If you want something that looks right for an old photo, up to about the 1920w, split the render into channels and just use the blue. Unfortunately, filters rarely match the RGB split of a CGI image, or what a digital camera records. Yoo may need to reduce the blue brightness a bit and include a little of the green channel. By the 1940s, film could respond to red light, but you still have the excess blue-sensitivity. If you're using a Clouds shader for your sky, you might do better to use grey instead of blue for the base colour, which would give the chief effect of the yellow filter. Without it, clouds tended to vanish. Incidentally, modern chromogenic films use the dye immages of colour film, but still try to match the behaviour of traditional black and white. It's what photographers expect. There are other differences besides the colour response.