Skin
Skins are graphic files in The Sims,The Sims 2, and The Sims 3 which are used to represent the head, body and clothing of a Sim, but how they work in the three games is somewhat different. The wide availability of third-party skins has unquestionably contributed to the high visibility of the series.
The Sims
Skins in The Sims are comprised of two parts per Sim: a head and a body. Both are simple bitmap images, numbered to correspond with the mesh file which gives them the desired shape on the Sim. Each image file is that Sim's entire body or head, including skin (there are 3 skin tones - light, medium and dark - and it is common for the same outfit to be made in all 3 tones) and hair. However, a Sim's hands are separate, and hands are not part of a body skin, though it is possible for a skin to have custom hands. For example, a skin might have white hands so a Sim using it will appear to be wearing white gloves. Some skins have accessories such as hats and glasses, which have separate BMP and mesh files.
True custom skin tones aren't possible, but variations of the basic skin tones are, as long as they're named so that the game engine "sees" them as light, medium, or dark tones. Some creators made "pale" versions of the light skin tone and made them available for use by others.
Skins are available for different modes of dressing: Everyday, Pajamas, Swimwear and Formal. Some jobs also have "uniform" skins. There are also nude skins for when a Sim is using a toilet, using the Vibrate interaction on another Sim or is taking a bath or shower. (a censor blur appears on top of a naked Sim).
Unlike The Sims 2, The Sims does not distinguish between custom skins and those included with the game. All head skins and Everyday skins are available when aging a baby to a child, aging a child to an adult with the Age of Instant charm, or generating a townie.
Once created, a Sim's appearance is not editable from within the game, except for their clothes, which can be changed by using a dresser. Some third-party utilities let players change a Sim's appearance.
The Sims 2
With the advent of the full 3d environment in The Sims 2 (See Comparison between The Sims and The Sims 2), skins become more complex. Skins are still in BMP format but are now contained in a "package" file, which can be extracted (and compiled) with the included Body Shop tool.
A Sim's body is now separate from their clothing, and there are now 4 basic skin tones (light, medium-light, medium-dark and dark) with each skin tone's graphic files being divided by age and gender. In addition, custom skin tones are now possible, and many are available as custom content. That Sims' bodies and clothing are separate makes all outfits available to all Sims of the appropriate age and gender, regardless of their skin tone or body shape. It also means it is now possible to create distinguishing marks such as tattoos that are unique to a Sim, although in practice this is not an economical use of drive space.
Hair is also separate, with different colours and styles, and eye colour can also be set. These traits are carried in a Sim's genetics, and passed on to their children. While The Sims 2 generally distinguishes between custom skins and those included with the game, hair is a partial exception. Some hair creators place their creations in their appropriate color bins. Hair that has been "binned" will still be marked as custom content, but it will not be treated as custom hair, and the game may use it when assigning hair to a newly generated Sim.
Sims can change their appearance through a mirror. Hair colour, hairstyle, facial hair (male teens and older only) make-up, glasses and face paint can be changed.
Clothing has also undergone changes. Not only is there far more detail, but when wearing their Everyday clothes the top half of a Sim's outfit can be made separate from the bottom half, or simply be one piece. The modes of dressing are Everyday, PJs, Underwear, Swimwear, Formal, and Gym Clothes/Workout Gear. Seasons added Outerwear, which will usually be worn in cold weather. Pregnant Sims will wear maternity clothes, although this can't be assigned by players.
The Sims 3
In The Sims 3, players can choose from one of the 6 base colors offered and then use the sliders for a more customized skin tone. Custom skins are also available. Base of game doens´t include aliens and vampires as life stage but it is completely possible create them with pale green tones and pale blue tones.
