Skeletal muscle fibers are not all the same. Traditionally, they were categorized depending on their varying color, which is a reflection of myoglobin content.
Red Fibers: Those containing high levels of myoglobin and oxygen storing proteins had a red appearance. Red muscle fibers tend to have more mitochondria and greater local capillary density.
White Fibers: Those with a low content had a white appearance.
As more was learned about the functional differences between skeletal muscle fibers, they were also classified, depending on their twitch capabilities, into fast and slow twitch, traits that largely, but not completely, overlap the previous classification based on color.