Natural Family Planning did use a red and green color scheme.
The problem they encountered was that red and green had different connotations depending on whether you were trying to conceive or avoid pregnancy. Do you use green to show fertility(trying to conceive) or did you use it as a go ahead for sex w/o concern about pregnancy?
For the record, NFP is not the same as the rhythm method where you count days and estimate your fertile times. NFP requires meticulous tracking of fertility symptoms and a commitment to abstinence during fertile times.
I was conceived due to a misunderstanding about how the rhythm method of birth control worked.
My mom later told me there's a medical term for a woman who uses the rhythm method of conception control -- Mother.
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