A method of determining the phase of an estrous cycle that a mammal is in at a given time that a biological sample is obtained from the mammal is provided. The biological sample obtained from the mammal is combined with at least one flavonoid pigment that reacts with the biological sample on a hydrophobic surface to provide a color response. The estrus phase of the estrous cycle has a corresponding color response that is distinguishable from the color response of each other phase of the estrous cycle to an unaided human eye. The corresponding color response is correlated to the estrus phase of the estrous cycle.
