Are fairies good or bad? Can you tell if a fairy is benevolent by their color? Mostly not, is the answer to both. Those who believe in the world of the elements, sprites, and dark fairies also believe that there is nothing good or bad except what you make of it, and what are commonly referred to as "dark fairies" are simply fairies that are going about their fairy business.
The world is full of fairy (or faery) lore wherein these darker elements coerce human children and sometimes male adults to follow them into their watery homes or their neck of the woods, never to be seen or heard from again. These dark fairies keep the children hostage to raise them as their own--sometimes replacing them in their cribs with their own, deformed babies, hence the reference to "changelings"--or keep the adult males for entertainment. One of the best dark sprite stories is the famous "Darby O'Gill and the Little People.
Colorful fairies are the result of Hollywood, Disney, and anime. "Real" fairies are often light or dark skinned and dress in colorful tunics, but all of the winged ones have simply transparent or translucent wings, and not the blue fairies or purple fairies of a child's nursery walls or the latest bumper stickers. As whimsical and pretty as they are, these rainbows colored fairies or purple fairies as depicted on some of the gothic inspired websites are not particularly reflective of true faeries. That isn't to say that they aren't beautifully drawn and interesting.
Most fairies have a mischievous side, it's in their nature. Depending on where you live they can be helpful or spiteful if you get on their wrong side, but inherently good--bright--or evil (dark fairies)? That is man's burden alone.
If you love fairy illustrations, you will enjoy "Good Fairies Bad Fairies," a beautiful book full of color illustrations and faery lore. Both "Good" and "Bad" faeries are included in this whimsical book as well as all kinds of information regarding the mystical characters. Find out how to name a faery, the different fairy classifcations and much more.