This seems like a really fun problem and I think you found the solution with the best cost/effort ratio. The first thing that sprung to mind on seeing the problem for me was to use the fact that products often come in several colour variations but use very similar images for each. If you mask out the parts of the image that are similar you can get the single 'actual' colour as it might be labeled by a human. This would also allow you to search for neutral tones like grey and black.
And seeing as you've gone to all the trouble of figuring out how to grab dominant colours from images, why not allow users to upload an image of their logo and grab the dominant colour from that? This could help avoid the problem of having noticably different shades, without making the user do anything technical like searching with an RGB value and a threshold.
And seeing as you've gone to all the trouble of figuring out how to grab dominant colours from images, why not allow users to upload an image of their logo and grab the dominant colour from that? This could help avoid the problem of having noticably different shades, without making the user do anything technical like searching with an RGB value and a threshold.