You want the audience to read the slide as you are talking, they shouldn t have to concentrate to read it. Do not put up a slide of text and then read it to the audience. If it is readable, they ll read it, if it is not, shame on you for presenting it. Use a solid background color. If a gradation of color is used, the text must contrast with all gradation of the background. .
Medium blue is good for a background with the text in white, bright yellow, pink, etc.
Do not use dark red, dark green, or dark blue. They do not project well. If you have lists, it is best if the items come up one at a time and the previous items change to a different color. The great thing about PowerPoint or other presentation applications is that they can be changed easily. The best test is to do your slide presentation, in advance, in a semi-dark room.