Conditional Formatting will not do it. There is an old technique using Global Repeating fields that might work. I did not find it with a google search. Maybe someone remembers the technique.
So far the closest thing to my objective has been to use the TEXTCOLOR function and create a calc field applying the color value form my Colour Field and applying it to some text in the calculated field. so now at least I have one field in each row that has a different colour. Not ideal but close .
You cannot change the color of a graphic object. but you can stack several copies, each of a different color on top of each other and control which one is visible.
You can, however, make a button's fill color change as controlled by a conditional format expression and you can get it "close" to an oval. Close enough? Depend on what you want to set up.
This is easiest with FileMaker 13 where you can use the "hide object when" feature to control which copy is visible.
In older versions, you'd put each object on a different panel of a small invisible tab control and use a script to control which tab panel is in front and thus which graphic object is visible. This would only work in form view.
Thanks - that's brilliant! Conditional formatting of a button will work well for me.