Atilla,
I had been thinking the same thing as you...occasionally you see lots of locales pop up that are "late yellows" that will go green. I was (And still am) worried that 'other' animals would be sold as canaries and that we would get lots of contamination into what people were hoping was new blood.
From studying as many pictures as I could find and looking at my animals (4.4), I have this thought...from what I know (and I am certainly not an expert), all locales change with a diffuse greening that comes in all over, frequently starting on the head. Many scales at once turn green at any point on the scale.
I have not had any canaries change, however, 3 of my animals are "dirty" canaries. Here is my guess/how I personally would evaluate future animals. The A grade yellows have a few single solid green scales (see my page www.canarychondros.com and look at the 3rd pic of sunflower). If I look at my dirty ones, I can see where they used to have their earlier green sclaes, they are just not as well seen as with the solid yellows. In my mind, if I couldn't find these 'spots' I wouldn't buy the animal as a kafiau/canary. The difference being that no other locale seems to have single solid green scales and then add diffuse green. I do have one really good pic I'll try to add to my site.
Now, if anyone thinks I am an idiot, or if I have gotten something wrong, please tell me as I'd rather correct my thinking than make a mistake.

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