Hi John alcohol, beard oils ,leave in conditioners and water that is extremely filtered can cause purple. While it is rare, if you get a lot of alcohol on your hair even days after it can turn purple.

If you rinse the dye out and see purple, red or green this is normal, it will turn brown over night. Ironically I have dyed my goatee for 7 years and even using the same pack I get different results after I rinse, however it always turns dark brown (almost black) on me overnight. Beer, mixed drinks, aftershave etc can cause purple. You also have to watch the shampoo you use (one with out oils in the ingredients are best). I personally use Pert, even though it is a all in one shampoo/conditioner it has no oils. Also make sure you do not forget to shampoo 24 hours later after you rinse the dye out, if not it can oxidize and turn colors. If it should ever turn purple you can go put some on for 30 minutes and correct it. If you are doing black, you can even go over purple with natural red henna and it will turn it black again just leaving it on for 15-20 minutes. If you are prone to getting purple ( which could be the water in your system, some is filtered more than others) then you can switch to a mahogany henna hair dye, which has a slight more amount of red and will fix the problem. You can also mix mahogany with any of the browns (or any color for that matter)