On the Object Palette under the Field tab there is an appearance property. Open the drop down and choose Custom. Under the background fill make sure the Style dropdown is set to Solid. Now you can click on the paint can and choose which ever colour you want.
Thanks, Paul, for your reply.
However, I would like to change the colour of the dot once an user has selected the radio button. the defualt colour is black.
The process you've mentioned is to change the background colour of the circle.