Color is a very poweful tool, but you have to be careful about how you use it, especially with text. Avoid using complementary colors (red/green, blue/orange, purple/yellow) as text and background colors. It is much easier on the eyes to use light text on a dark background, or vice versa.
Color is useful for emphasizing and focusing your viewer's attention. In the slides below, one's eye is immediately drawn to the colored text and the color photo first.
You should be conveying most of the information for your presentation, not your slides. Your slides should highlight only the most important information. If the information doesn't fit, then it's probably too long! A good rule of thumb is no more than 6 words a line nor 6 lines a slide.
Also make sure you are using a font size that is large enough for an audience member to see, regardless of where he or she may be sitting.
It is important that you stay consistent with your fonts. Use the same fonts for your title and body text throughout your presentation. And don't use more than 3 fonts!
Use fonts that will be legible when projected on a screen. Your audience needs to be able to read your text whether they are sitting close or far away. Also, use text that is appropriate to your subject matter. "Comic Sans" is almost never a good choice for a serious presentation.
Many designs, in web and print, are structured along a grid system. Use the rows and columns of a grid to align objects and maintain unity from slide to slide.