1. A system for determining the blood content within the gastrointestinal tract, comprising: a data processor configured to: receive data captured by a swallowable in-vivo device in a gastrointestinal tract, the data comprising a set of in-vivo images of the gastrointestinal tract; compare values of an image from the received set of images to a reference value of blood, the reference value calculated from sample images known to contain blood; and based on the comparison, generate a probability indication of a presence of blood in the image; and a monitor to display a color video showing the presence of blood.