Pick a Color, Any Color
You might have noticed that, for the most part, the bad guys have red lightsabers, and the good guys have lightsabers that are … not red. According to the various novels, TV productions, and other lore that fans have obsessed over for years, there is symbolic meaning to the color of a Jedi's or Sith's lightsaber.
Yeah, that's right. We're getting our nerd on. If a slight Lucasfilm obsession (and random Wookiee noises) upsets you, it's best that you turn away now.
You stayed? Well chosen, young Padawan.
It all has to do with the lightsaber's basic construction. Typically, the blade is created by manipulating the Force through specific crystals, which have been imbued with special properties through an initial, intense period of meditation.
Because they have been known to create stronger "blades," synthetic crystals are the preferred choice of the Sith, whereas the Jedi prefer crystals from the Ilum Caves. These natural crystals produce colors like blue, green, and the less common purple, yellow, and orange. (Source)
According to the official Star Wars Wikipedia, Wookieepedia, different colors are used to indicate the class of Jedi:
Blue indicated a Jedi Guardian, a Jedi who used the Force on a more physical level. Green indicated a Jedi Consular, a Jedi who preferred to reflect on the mysteries of the Force and fight the dark side at its heart. Yellow indicated a Jedi Sentinel, a Jedi who honed their skills in a balance of combat and scholarly pursuits. (Source)
But, even if you don't do research on the reasons why different characters have different colored lightsabers (it's not really explained in the movies, that's for sure), there is a consistency to which George Lucas adheres.
Blue lightsabers and those who wield them tend to prioritize justice and protecting others—warriors like Obi-Wan Kenobi and Anakin Skywalker. Blue is often a color used to represent steadfastness, confidence, and loyalty, so it's no wonder Lucas would've picked it for these heroes to brandish.
The most famous green-lightsabered character is Yoda, which is perfect, and not just because they match. He is known for being "one" with the Force, in tune to nature and all that surrounds him, so what better color could he have? And, just like Wookieepedia mentions above, he definitely falls into the "Consular" Jedi category, in that he typically prefers to negotiate and diplomatically seek out a resolution before having to use the force of the Force.
In Revenge of the Sith, we even get to see Mace Windu in action using his rare purple lightsaber, which could mean that he suffers from moral ambiguity. What do you get when you mix red (Sith/bad) and blue (Jedi/good)? Yup. Purple.
Interesting fact, though: although Mace's character indeed experiences some moral dilemmas, the reason his lightsaber is purple is because Samuel L. Jackson required that as a stipulation to appear in the films. Purple is his favorite color, and it would help him stand out among the other Jedi. (Source)
Finally, we get to the crimson blades that are preferred by Sith Lords the galaxy over. Red has long been a color used to represent the Big Bad because of its intensity and allusion to blood. The devil is also often depicted in red, and since he's considered the worst baddie around, it's just one more strike against what is really a lovely color.
So, not only do the colored lightsabers help to distinguish whom we want to root for during a fight scene, they also help characterize the people who use them.