Redness of the skin is the first visible result from spanking. The redness comes from a widening of the capillaries (blood vessels) in the dermis (middle layer of the skin). The harder and longer a bottom is spanked, the redder it becomes - up to a certain maximum.
This effect is related to blushing in the face, which comes from embarrassment or from physical exhaustion. Typically, a person who is subjected to a good spanking gets not only a red bottom but also a red face.
In addition to the change of color, the redness can also be felt - as temperature. The increased blood circulation makes the spanked skin warmer to touch than the normal, unspanked skin. This is why a spankee will not feel cold in their bottom during a spanking even when bared for a longer time, whereas other parts of their body, such as their back, legs or feet, may become cold with time.
Using redness as a measure
The redness effect is psychologically impressive for both the spanker and the spankee and one of the things that make (consensual) spanking and getting spanked fun. It is also sometimes used as an approximate indicator of how hard a spanking was (or when it is enough). However it should be noted that some bottoms redden more easily than others. Also, many other factors have an influence: for example, the time of day, or whether the spankee has consumed alcohol or other blood thinning drugs such as aspirin.
For people with a dark skin color, the redness effect is much less visible than for people with few pigments in their skin, although the capillaries widen the same.
Redness in art, film, and literature
In spanking stories, spanking art and spanking videos, the redness effect is often exaggerated. In stories, well-spanked bottoms are frequently described as "cherry red", and some artists draw the bottoms of their spankee characters in such a cherry-red color, especially in comic and cartoon-style drawings (for example, Comixpank or Jay Em). In reality, the redness is more of a pink than a deep red tone because (a) there is always space between the capillaries even when they are widened, and (b) because the blood-red color of the capillaries is overlaid by the semi-transparent yellowish-white epidermis.
In spanking photos created for magazines (and occasionally in videos), models sometimes apply rouge make-up to avoid the need for an actual spanking or to create the illusion of a severe punishment session. More recently, various visual media have created these enhancements using digital computer graphic techniques. For example, the Italian Spanking Art Pages offers a (non free) tutorial on how to realistically redden a bottom as if it had been spanked using Photoshop.
Rendered art by Mayhem.