You can't stain your genitals with your own feces. It's not full of die or pigment, it washes off. You may have a skin condition like ezcema and when we have something like that, it can discolor the skin. Hyperpigmentation is also common. https://www.healthline.com/health/why-is-my-private-area-dark Hormonal changes and irritation and friction most often cause it and it is normal. Avoid tight fitting clothing, moisturize it (non scented, gentle products only), avoid hair removal, wear sweat wicking clothing. One thing to check into is acanthosis nigricans. If it is bothering you, see a doctor. It's not feces stains though.