* How will you check if the henna paste used has chemicals ?
Usually the henna plant takes few hours to leave it's  ORANGISH color on your skin (about 4 hours) If the henna paste gives you the DARK RED color with in minutes / less than an hour, It may have hair dye added.

The real ONLY henna paste will take 1 or 2 days to get the RED or blackish color.
Various artists add different ingredients to the henna paste - Eucaluptas oil, Vicks, tiger balm, tea, coffee, Petrol, red wine, lime juice, etc.
The color also depends upon the body heat and the climate.