The quality about gemstone color is identified by their inclusions.
I am explaining you more precisely.You have two categories : colorless gemstones and colored gemstones.
The value of colorless gemstones is considered on a scale of the most colorless till the color. We used GIA scale with letter. You would have to understand that more it is colorless more is expensive. For the knowledge you need to have the certificat about your gemstone.
Now, the value of colored gemstones is considered too on a scale. B being the worst and AAA being the best.