When I was approached during the Fort Lauderdale show with regard to identifying a stamp, I gave the collector my best “OPINION”. That is all that it was – an opinion.
One of the most difficult determinations in philately are shades. In the early days inks were mixed by something approaching “a bucket and spade method”. Quality control devices as we have today, just were not developed back then. Canada’s Small Queens, or Britain’s Edward VII low values are prime examples of the variation of shades available to be collected. Apart from the listed shades found in the catalogues, there are many other “sub shades” as well. Then there is the issue of oxidization, particularly with the red inks, to contend with. Some colors just fade with time, are affected by humidity or heat, and so on. On top of that my eyes see a shade of say bluish-pink differently to your eyes.
The background light source is also a factor.
So for the collector with a potentially valuable stamp, show it to a knowledgeable collector in the local stamp club. If that person believes the stamp to be a good example of the valuable variety, it may be wise to seek an endorsement from a recognized authority and obtain a certificate of authenticity. The authority will charge for this service. When you read the certificate it will still employ the word “OPINION”!
As an aside, the detection equipment available today and used to discover altered stamps, forgeries, etc. is so much more sophisticated than, say, twenty or thirty years ago. Even if I obtain a stamp with a good certificate from a recognized authority and which is dated in the 1970’s or earlier; or even early 1980’s, I take the precaution of submitting it & requesting a new certificate. Such action makes me feel confident that I will be selling a valid stamp, even though that word ‘OPINION’ will still appear on the new certificate!