For my IB chemistry extended essay, I'm investigating the effect lemon juice has on the color intensity of henna, an olive-green, tooth-paste like substance we apply to the body for either decoration or health reasons (image google it). I know that the lemon juice darkens the henna, but I have to carry out an experiment that validates this so that I can then explain the molecular reactions that happen when these two substances come together. Do you have any ideas of an experiment that's fairly accurate and gives concrete data values for the color change that henna may undergo after the lemon juice has been added. I was thinking of simply 'painting' henna across some animal skin and leave it dry and see the color change may happen, but this is fairly bias...I would rather have proper data values. Any ideas?