Floppycats
It takes weeks, maybe even months for the change in color to be perfectly visible because the process is gradual.
The limits of the chemical reaction behind the pigment production process which depends on this temperature, are also different from one individual to another.
Diet deficient in the amino acid thyrosine and/or phenylalanine causes the hair color to change from black to reddish color with a reduction of the pigment melanin in the hair.
Lots can, including: The weather / The temperature in the house (Environment Temperature), Fever, Acute Inflammation – Local or General, Stress, Digestive Disease and Dermatological Issues.