There is a myth within the cannabis community that the quality of your bud can be revealed by the color of the ash as you smoke it. If the ash is a silver/white color it is considered to be properly cured, full-bodied, fire flower. However, if the ash comes up black this could be a sign of flower that was improperly grown, flushed, or cured (some people out there make an argument towards this being a sign that pesticides or other contaminants are present).
Have no fear - if you have smoked cannabis that produced black ash you are not going to face imminent death. In fact, there is something relatively simple going on here: and the answer is; water.
When there is black ash this is likely caused by water content within the leaves. As water burns within the plant it creates a black smoke. Think back to that time you were at a bonfire and someone threw fresh wood or kindling onto the flame, the embers pop and crackle while the smoke spirals out from the pit in a black plume.
So perhaps the bud was harvested too late or too early, and dried too quickly. There are a number of errors a grower might make with processing that could lead to extra water content. However, if dried at a humidity around 45-50% and Temperature between 55-65 Degrees F you should avoid the black ash effect.
It is also possible that when your joints burn black that there could be more oil or carbon burning off of the plants. Terpenes are hydrocarbons, and many of the oils and flavonoids present in cannabis are volatile under flame. This can cause them to burn black or leave a black residue. Even in a silver, light colored ash you will still see some particulate that has burned black - this is common when burning cannabis because of how resinous its flowers are. You may see this in other herb varieties too like lavender or catnip because of their oil content.
Now, what's with the white ash? Is it a good thing?
When combusting, different elements will burn different colors. We already covered carbon above, which may burn the blackest out of all the elements we currently burn. When the black ash is still present, this is because there is still more carbon to be burnt (coal is black for a good reason). But what makes white ash? White ash is derived from various elements found within the flower such as magnesium that burn white. Depending on the breed and grow practices you may have more or less white ash present. Ultimately, white ash occurs when all of the carbon has reacted and converted to carbon oxide. A properly flushed plant will produce ash that is far more uniform, verses a plant that still has any residual nutrients or compounds within the tissue.
Flushing isn't always necessary depending on your grow practices, when it comes to achieving a lighter burning ash. If you grow hydroponically or in a no-till style, there is no need to do a 1-2 week flush before harvest. If you are using a decent amount of additives that require processing within the roots and vascular system, you may want to consider a flush (especially in soil or coco). The plant is capable of using up all of its nutrients within a 48 hour period, and most of this purging will happen in the drying room. So preserve your trichomes and plant vitality by chopping before your flush if you have the right grow practices.
If you are concerned about getting all residual nutrients out of the vascular system I would recommend stimulating your plants by bringing your temperatures down to about 65 Degrees F and using various applications of root stimulation to help with priming the vascular system for harvest.
So whether white ash or black ash are a telltale for quality or lack there of, I think it's important to realize there are multiple factors that contribute to ash color. Sometimes the most oily bud can produce black ash, and sometimes the most flavorless flower can produce white ash. More importantly - focus on moisture content, curing humidity, and planning out your flushing based off your unique practices.