If you take picture in e.g. jpg format, you will receive 8 bits per color channel. Tiff is much better in this point of view. It is better to take picture to RAW format and process it to tiff with more bits per channel:

http://laurashoe.com/2011/08/09/8-versus-16-bit-what-does-it-really-mean/

It will result in better ability to distinguish coloration. Good source of light is also necessary (optimal temperature of light around 5500 K).

Gimp is not well for such experiments because it works with 8 bits per color channel only.

Tutorial for determination of color code in e.g.:

https://www.youtube.com/watch?v=VHyFgTZmnF8

Thank you Miroslav. this is a very good and informative hint. yet, i am afraid i did not get how to estimate or determine the percentage between the colors. Photos are already taken in sun light in jpg format. i read in some articles that the percentage between colors is determined in histogram but i could not figure it out. any idea?

Thanks again

Hello Maha, I think that the color picker toolbox will be the best for your use. You can find instruction to how to determine color using GIMP and the toolbox in links:

http://www.instructables.com/id/How-to-get-the-Hex-code-for-any-color-on-your-scre/

http://docs.gimp.org/en/gimp-tool-color-picker.html

Sincerely yours, Miroslav

Hello Miroslav. thanks indeed for prompt response and concern. i will go read and watch the videos in details.

Regards, Maha