There are two ways to go when designing a postcard:
1. Relate the design closely to your brand. This can help to build a small business's overall brand recognition. But, if your brand has a very clean, light look and color palette, then your postcard might get overlooked in the mail.
2. Design the postcard first to be eye-catching. This doesn't mean you shouldn't match your brand at all, but instead that you should think first about the viewer's reaction when pulling the card out of the mailbox. Make sure you capture the viewer's interest with the look and offer, and then match it to your brand secondarily.