I have a white printing on LDPE which is yellowing. How can I determinate the yellowing in a datacolor instrument?
I would suggest simply using your instrument to measure CIELAB values and examine the L* and b* values. As it yellows, your sample will get lower in L* (lightness) and higher in b* (yellowness-blueness). With some experience, you will be able to establish tolerances for changes in these dimensions that are unacceptable for your application. There are also whiteness indices that you might look into. The software with your instrument can probably calculate them. The only advantage of a whiteness index is that it might give you a reasonable single number to look at.