Printing sandstone with colors can be challenging. In my experience, some colors come out brighter and some colors come out darker than expected. Also there are some colors that don’t match at all what you expect.
I needed a color reference to make the printing results more reliable. I found these full color sandstone samples on Shapeways and printed them for about $9. Fortunately, the original model file is available for download. It includes the following image map:
The printed sandstone samples consists of 5 small colored bars:
I took the original image map and used Gimp to determine the color values used for the print. I then collated the hex values of the colors in a spreadsheet. You can download a CSV version of the spreadsheet.
Now I can look at the printed sandstone samples to see which color I want. I then find the matching color hex value in the spreadsheet and use that for coloring the 3D model in Blender for printing.