Hello Youcef,

If you have access to a portable HunterLab such as MiniScan EZ 4500 L it is easy to measure L*, a* and b*, with dimension L* for lightness.

a* in negative values indicate green while positive values indicate magenta, b* in negative values indicate blue and positive values indicate yellow.

There are certain equation to measure e.g. whitness using the a, b and L values.

https://www.hunterlab.com/benchtop-spectrophotometers.html

Thank you Amin Shavandi for your suggestion. Unfortunately I don't have access.

Yellowness index is widely used to assess platics degradation and aging

Article Photodegradation and photostabilization of polymers, especia...

http://www.ampacet.com/usersimage/File/tutorials/Antioxidants.pdf

Article Stabilizations of Molecular Structures and Mechanical Proper...

https://www.xrite.com/service-support/whiteness_and_yellowness_indices_in_a_spectroeye

Article Photostabilization of poly(vinyl chloride) – Still on the run

https://www.mddionline.com/improving-gamma-resistance-flexible-pvc

http://web.ist.utl.pt/ist10800/Artigos/2008_A85.pdf

http://gost-snip.su/download/astm_d_6290_05_standard_test_method_for_color_determination.

http://zeus.plmsc.psu.edu/~manias/PDFs/PVC-Polymer09.pdf

A low tech solution could simply be to use an RGB colour chart, bu this may not be sensitive enough as it relies on visually matching colours. See http://www.rapidtables.com/web/color/RGB_Color.htm

Dear Gyorgy Banhegyi

Thank you for the useful linked you have shared.

Dear Costa Conn,

I thought about RGB too. Thank you.

Hi,

if you have to use just RGB you can also check the web http://www.cvrl.org/ , where you will find an annotated library of downloadable standard data sets relevant to color and vision research.