The default colors are blk/wht (which is a pain
IMO).
With an object selected and you click on the gradient button (at the bottom of the tools palette) your object is automatically filled with a blk/wht gradient.
To change these colors click on the gradient preview in the gradient palette, then the color stops will appear.
Then you must change the color mode in order to use color(crazy isn't it!) Go to the color palette click on that little arrow button and choose what color mode you want to work in (ie. RGB,CMYK etc.)
Then click once on a color stop you want to change, Now here comes the crazy part... if you click on a color, either Color bar or Swatches the entire object will turn to that color! RATS!
You must hold the ALT key then click on a color to only effect that color stop!!
Just my 2 cents ;\