Because it is a different colour, meaning that all the training you received learning focussed on different shades of pink no longer applies. Darker skin means

Because it is a different colour, meaning that all the training you received learning focussed on different shades of pink no longer applies. Darker skin means the symptoms you are looking for are lower contrast and a different colour. It’s a simple case of repeating all that training against different skin colours. It’s often difficult for someone trained exclusively with pale skin to recognise something as obvious as sunburn on darker skin, for example.