Like you, I've only seen white boxes for balloon bouquet deliveries. Have you tried contacting some of the wholesalers listed at the Qualatex Distributors Page? There might be one who offers coloured variations of the boxes.
Or you have the boxes made for you. If you search for 'custom made delivery boxes' on Google, you'll find lots of companies offering this service. AscDirect.co.uk for example offers colour printed boxes.
