Luminance and how it relates to color

- In Photoshop and other graphics programs,…brightness goes by many names including shade,…tone and my favorite, luminance.…And an everyday average RGB image,…which is far and away the most common…kind of image you'll encounter,…luminance is measured in steps known as levels.…A level of zero is black and a level of 255 is white.…The other levels, one through 254,…represent the progressively brighter…shades of gray in between.…So where does color come in?…You have three luminance only versions…of an image called channels.…

One each for the primary colors of light,…red, green and blue, hence, RGB.…The three channels are respectively colorized…red, green and blue and then blended together.…In other words, color is a function of luminance.…Meanwhile, luminance and color sometimes…react with each other in surprising ways.…Look at a strip of gray values…and you see a steady progression…of neutral levels just as you would expect.…Throw a cool color like blue into the mix…and the blues go from light to dark as expected.…