Put it this way. If you have 2 pastel ball pythons. One of very high quality, and one of very low quality. 100% of the time you will know which is which. You just know. It's more eye catching.
Now you have some morphs that are visually very normal looking but with subtle differences such as yellow belly. A high quality yellow belly will more prominently display those subtle differences. It should be easily identified if you are familiar with the morph.
Within every morph are the characteristics that identify the morph. Some people prefer certain characteristics to stand out more or occur more and this is where the personal preference comes into play.
I believe the measurement of quality firstly comes from what a majority of people in the hobby like.
Secondly(though more importantly) it comes from what you prefer and want in your animal
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