* By mineral: Quartz, the most abundant mineral on Earth, is often clear, white, or milky, but can be found in many other colors. So, if you're considering the most common mineral, clear/white would be a good answer.
* By type of crystal: If you're looking at the most common color across all types of crystals, it's hard to say, but clear/white is again likely to be high on the list due to the prevalence of quartz and other minerals that form clear crystals.
* By collection: The most common color in any given crystal collection will depend on what the owner has collected. It's likely that the most common color in a collection will be the color they find most appealing.
Ultimately, there isn't one definitive answer to this question!