Radio buttons can be used like checkboxes but are really designed when you want the user to choose just one item from a selection of options.  Add this just under the checkbox div element:

<p>What is your favourite colour?
    <label for="blue">Blue </label><input type="radio" name="colour" value="blue" id="blue">
    <label for="green">Green </label><input type="radio" name="colour" value="green" id="green">

Note that the two have the same name as only one will be selected and sent with the form but they have different values (and id attributes).  Save and try to select both colours.

Again you can have one of the radio buttons automatically selected by adding the checked attribute.