I use the term Liqueur very vaguely on this site and even in the strictest definitions the term covers a lot of ground. Basically a liqueur is a sweetened or flavored alcoholic beverage. There are many further ways to categorize them and according to different countries this is done differently. On this site I include cordials, cream liqueurs, fruit liqueurs, and herbal liqueurs here. The base spirit can be fermented from anything that is fermentable.
Varies greatly dependent on style and ingredients.
Liqueurs allow for many different flavors and colorings, even artificial ones. As a result the color spectrum's only limit is the rainbow itself.
Anything you can imagine.
Strength of liqueurs varies greatly. Some as low as 10% alcohol by volume and ranging all the way to high proof spirits in the 60% range.