This question will forever pop up over and over. Every lab has there own system of top colors that they use to distinguish the quality of their product. For example, XYZ lab sells blue tops, green tops and red tops. They allege that the blues have 96% purity and cost $100, the greens 97% and cost $200, and the reds 99-100% and cost $400. Another lab can have an entirely different system. You need to learn what labs you trust and what quality they have for various colors. One labs greens are not another labs greens.
IN SHORT: cap colors are how a lab differentiate their product quality.