A photographic subject is illuminated by light from everywhere around it; natural sources, camera flashes, and reflected light that has bounced off of every colored object nearby. Your eyes automatically compensate for this and consider the "whitest" part of the picture to be true white. Cameras do this too, by adjusting the colors so that the average is uniform gray, but the process hurts you if you're trying to represent true color.
If you turn off your camera's auto white balance feature (AWB), you will probably have to choose a lighting mode, to tell the camera whether you're shooting under a blue sky, a yellowish tungsten lamp, or a greenish flourescent tube. There could be other sources, like flash for example. But then your camera will not mess with the colors and as long as you have chosen the right lighting the picture will be very very close.
To color-correct your monitor, turn on the lights in your room and put a white piece of paper next to the screen, then open a "blank" window and expand it to fill the screen with white. You can then adjust your monitor's color balance or "color temperature" setting to match the paper. You shouldn't use a monitor near a window if you intend to do color-accurate work with it, and you should never use a monitor in the dark, because it will make your eye shift its color perception every time you load a new image.
Have you ever used a Color Checker? It's a rectangular chart of standardized color swatches used by photographers and printers to make sure that colors look as close as possible to "the real thing." You can buy one for about $40-$100.
https://en.wikipedia.org/wiki/ColorChecker
The way you use these is, you photograph the Color Checker with the subject (or put it right in front of the subject) in a few frames, and then in processing you verify that the swatches look the same. On the back of the card you'll find the RGB/CMYK values that correspond to each swatch so you can check them in PhotoShop if you like. This device can help you get the colors spot-on.
Unfortunately when buying items, there is no way to know if the seller has used the correct settings, or if they made any attempt to correct color, so you can't "fix" an photograph that you see in a listing.
