It is possible to accurately determine the original colors of a black and white photo, even if the image has been converted from color to grayscale and may have undergone various adjustments and edits. One way to do this is by using software tools that can analyze the grayscale image and estimate the original color values based on the shades of gray. These tools can be found online, such as the Image color set calculator, which can count the number of colors in an image, find the dominant color, and create a table of colors that make up the original image.
Another approach is to use color checking tools, such as a color checker chart, to help determine the colors in the image. By positioning the target close to the subject or in the same light as the subject, and filling the frame as much as possible with the target, the color checker chart can provide a more accurate representation of the colors in the image.
Additionally, it can be helpful to research the colors that were popular in the area where the photo was taken, especially if it is a historical photo. This can provide clues about the colors that may have been present in the original image.
Ultimately, accurately determining the original colors of a black and white photo requires a combination of technical tools and research, as well as a good understanding of color theory and the historical context of the image.