When converting a color photograph to black and white, how can an expert specifically use the original color channels to darken blue skies while brightening green foliage?

When converting a color photograph to black and white, an expert leverages the distinct luminance information contained within each of the original Red, Green, and Blue (RGB) color channels to precisely control the resulting grayscale tones. A digital color image is comprised of these three channels, with each channel recording the intensity, or luminance, of its respective color at every pixel. When converting to black and white, the goal is to translate this color luminance data into a spectrum of grayscale values. To specifically darken blue skies, the expert understands that blue objects, such as a clear sky, are typically very bright in the Blue channel because they reflect or emit a high amount of blue light. Conversely, blue objects absorb red and green light, making them appear comparatively darker in the Red and Green channels. To darken the sky in the black and white conversion, ....