Hindu wedding adornment that's said to get darker the more the newlyweds love each other (5)

I believe the answer is:

henna

(Other definitions for henna that I've seen before include "hair colouring" , "Cosmetic plant dye" , "Stainer" , "Red-brown dye used for body decorations" , "Shrub used for cosmetic colouring" .)

I've seen this clue in The New York Times.

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