Henna haunts me!

Asslaam 'Alaikum!
Lately, I've beeen obsessed with working out the secret formula to making a rich, dark Henna dye! I just can't seem to get it...
I used to be an ace at henna designing, but I havent touched a cone for the past two years, need to brush up on my skills...
In the meantime, any resources/tips to achieve a dark stain, pls share!
Jazakumullah!