This is a complete reversal of what we are used to. From earliest of times, as a child with a box of crayons, we have grown to see color as something that we "add" to white (such as to a blank piece of paper). This is actually called "subtractive color" due to the fact of rather than adding color (with a crayon for example), we are decreasing the amount of light that is reflecting back to us from the paper's surface. We are subtracting light. If we were to subtract light evenly for all colors we would be yielding darkening shades of gray working our way towards black. If we were to subtract light for only certain frequencies of the visual spectrum, we would yield different opposing colors reflecting back to us from the paper's surface.
For cameras (film and digital), for scanners, for computers, etc. the colors we are capturing, defining, and reviewing are colors of light. These images are defined by using different shades of three primary "additive colors" - RGB (Red, Green, Blue). Through varying the values of these three colors of light, we can build a broad range of combinations yielding a seemingly endless selection of colors.
The negative view of this image (above) is another way of looking at this. Areas that are the darkest in this reversed image (such as the clouds) is where light is the strongest. The reversed image also shows the image using opposing colors (such as orange being the opposite of the blue, as found in the sky).
It is tricky to think of colors in reverse. It is tricky to see something that is white (such as the clouds) as being full of color (100% red + 100% green + 100% blue). It is difficult to think of the blue sky as the absence of red and green.
When adjusting colors for images, prior to preparing for print production, it is good to keep your images as RGB color values. At some point though this will need to be converted to a different "subtractive color" palette. The image file will need to be converted from RGB colors to CMYK colors. Printing equipment, printing with inks or toners onto paper, will need color values defined for the colors of inks or toners being used. "Four color printing" is with cyan, magenta, yellow and black colors ("process colors"). The images (below) show the different CMYK colors and once these are printed and layered one on the other, they will yield a printed photo with the colors blended.
The process for making this conversion may be as simple as making a specific selection within the photo/paint program being used for the image. Usually TIF files are used for images in print production, though there are also other image file types that can recognize colors defined as CMYK values.
For making adjustments to images, since this should be done prior to the conversion to CMYK, there are many different programs that could be used for making refinements (while it is in its original RGB state). For making the final conversion though, some programs are limited and may not provide the option of then switching to CMYK.
There are other considerations for refining images for production, and color images in particular, can be extra challenging. Other posts to this blog site have related to adjusting grayscale images though in time, would like to offer more information relating to working with color.
Photo source: One taken while cycling the roads of Cumberland County, PA, just northwest of Carlisle.