Any text that you can read easily and quickly is probably ok.  However here are some bad examples.  Try to avoid doing these things please!
Neither red on blue, nor blue on red are a good idea for color choice. Neither red on blue, nor blue on red are a good idea for color choice.
See what I mean? Not a good idea.  Very hard to read. See what I mean? Not a good idea.  Very hard to read.
Using a multi-colored or high contrast background can make reading the text almost impossible no matter what color you try to use. Using a multi-colored or high contrast background can make reading the text almost impossible no matter what color you try to use.
Also, picking colors that are very similar to the background makes reading quite difficult. Also, picking colors that are very similar to the background makes reading quite difficult.
Putting text on an animated background can be hard to read as well. Putting text on an animated background can be hard to read as well.
  And this is just wrong don't ever do this on a website, people's heads might explode!

Guidelines ~ Text in Guidelines