I believe it is an important part of female health tracking to monitor the color of the fluids. You can find different period blood color charts online and hopefully Fitbit uses female programmers to work on this, rather than males, because they have first-hand experience. I would also hope that very detailed descriptions are provided (and more medically correct than mine) to help both young & experienced women track this aspect of our bodies.
I would say the basic colors are:
Bright red (freshest blood the uterus is shedding, but sometimes seen during spotting)
Dark red (older blood, sometimes indicating end of period)
Brown/black (oldest blood, sometimes leftover from previous period or indicating end of period)
Pink (spotting, sometimes because blood has mixed with cervical fluid)
There are also instances of women having orange, gray, and even purple blood which indicate other related health issues and I think these should also be considered.
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