I have heard if you put on eucalyptus oil that helps make the color darker and i'm surprised lemon works because lemon normally bleaches things that's why it's a great thing to clean with or to brighten elbows and knees on your skin. I was at a gujurati wedding and they did that, the bride barely kept her mehndi on and it turned out really well. Also it was some no name party cone not a brand name mehndi that was applied.
when you take the mehndi off, rub vicks vaporub on it, and hold your hands over the stove (the stove must be on ofcourse). the heat from the stove and the vicks will make the colour very dark. I do this all the time, and it really works.
Mehndi on the palms of my hands always turns out darker than when it's on the back of my hand. I apply Vicks on the dried mehndi though and it always turns out dark, so that works for me.
my mehndi was not of good quality at all this year... it was the 'emergency' one which usually are good but not this time .. usually i just put mustard oil while washing it off after it dries, and then it gets darker after a few hours or so.
Or even I think oil in general....maybe one with a "garam effect"
I've heard that exposing henna to heat helps as well.......putting hands above stove or using a hair dryer, etc
And also that you should keep it on as long as possible...even if it has dried up. And that when you remove your mehndi....just rub it off...but don't wash with water. Apply vapor rub or oil instead.
I dont like how mehndi turns out on the back of my hand when the design goes down my arm. Its a gradient, from nice dark henna to pale. I was told its the heat of blood that brings out the colour and since it is the least in arms, colour is light there. And that if i dry it using blow drier, colour will turn out even:hmmm: