While black and white is a cheap option, it usually pays to go color. It can be tempting to design your own flyers, but the result is not always professional.
You’ll want to use high resolution graphics (avoid clip art!), corporate colors, clear, easy to understand language, and text in one or two fonts only, preferably. Avoid trying to cram too much information into your direct mail collateral, and ensure that font size is at least 11 point throughout to cater for people with vision difficulties.
Be sure to review and sign off on a printed proof before your direct mail goes to print, as colors often look different on screen.