PART II: COLORS AND TYPES OF WALLPAPERS

Soft colors
When making a wallpaper, I highly recommend using colors that blend for practical reasons. You don't want to give your viewers a headache, do you? Avoid strong, complimentary colors. On the other hand, you have to be careful and make sure the elements you want to point out are visible. Think of the fact that if a wallpaper seems bright or normal to you, it can look very dark or very bright to other people who have other settings for their monitors. If possible, change your monitor's brightness and see how others would see.

Contrasting colors
So, you want to make a wallpaper using contrasting colors? Very well. Choose your color combination, but be careful when doing this. You want contrasting colors, but they also have to be not too disturbing to the eye. Alternate colors so they don't clash and pay attention to how they look. Check every part of your wallpaper for "burn spots" which appear when you use strong colors overlaid on one another.

Inverted colors?
Around various wallpaper sites, you might have encountered wallpapers which feature, among others, characters with inverted color schemes. You can invert the color scheme of a picture simply by opening the image in Microsoft Paint and going to Image -> Invert Colors. This takes less than 10 seconds. A wallpaper you truly work on should have plenty of elements (or few, if you decide to keep it simple), but you must work on them! That will at least give you satisfaction that you did it right ^_^
(Not to mention inverted-color wallpapers look ugly.)

Different people, different styles
There are more styles but these are the most common ones. Here are some basic characteristics:
Grunge: this style is achieved using textures (especially grainy ones) and/or patterns. These are fitting for “action” or “evil” works.
Soft: wallpapers which are made of mainly filters and/or soft brushes are categorized
Celestial: the background is a starry scene, with clouds, stars, comets, planets etc. and (usually) an angelic character.
Vectored: these wallpapers require a lot of work and are among the best (quality-wise). Basically, vectored wallpapers can be modified into any of the aforementioned types so it’s a really flexible style.

Now comes the really important part, how to place elements and tie your work together...