This may be completely inappropriate for your situation, but you can force one of the buttons to be selected by default.

<input name="color" type="radio" value="red">
<input name="color" type="radio" value="green">
<input name="color" type="radio" value="blue" checked>
[download]

This sets Blue as the default value. The downside may be that

1. If the user doesn't notice, the wrong color may be indicated
2. The user cannot unselect a value for that button

This is often a bad idea, but occasionally useful.

-theo-
(so many nodes and so little time ... )
Note: All opinions are untested, unless otherwise stated