You can decide what colours you would like the Bin your form to be, by editing the settings for the Buttons in the Form Builder. In the Web Portal, go to Forms > View Form Templates > select the Edit Arrow for the form you want to change > Form Builder tab.
Inside the Form Builder, you can locate the multiple choice question with the buttons. Select the question, and then in the Field Settings, in the Items section, the choices will be listed with a coloured circle beside each.
To change the colour of the various choices / answers, simply click on the coloured circle, and then select the colour from the colour selector that pops up.
Once the changes have been made, Save Draft, and then Deploy / Publish Template. Any new forms created after these changes will have the new colours for the button options.