Your forms will take on the colours set at the account level for your Funnel.
However, you can control and update these colours at a form level from within the form editor. Select the form you would like to update from your Funnel Forms module, then select the three more action dots to get to Form Settings.
From within Settings select the tab titled Customise to see the colour selectors currently available on your account and applied to your form.
The active colour is indicated with a tick in the centre. You can change this to any of the other colours by selecting from the options shown for the Header colour or Accent colour.
If the colour you wish to apply is not displayed for selection, you can add another colour by entering the Hex colour code in the field available.
Remember to publish the changes you make here to see them live on your form.