Even though your potential customer will be reading it as a letter, you'll find things you like about it and things you don't, and you can make positive changes. Consider reinforcing your letter with a flyer or brochure if and only if your product needs one to highlight it. There are some products that look great in color, like landscaping. If you're sending information about how you're going to help people with their landscaping.
Sometimes a color picture will help -- but for many forms of direct mail it's not necessary. Make your letters as personal as you can. Don't write to some mailing list amorphous "Dear Potential Customer." Always write your copy as if you're writing to just one person. You may be sending out 10,000 letters, but you want each to feel like it was aimed right at the person who reads it.
Of course, today it's easy to personalize your letters, in the salutation and throughout the body; try that and see how people respond. Also, keep in mind that good direct mail copy consists of short sentences and paragraphs. And don't forget a call to action: tell them exactly what you want them to do. Use testimonials if you can. People expect us to be all gung-ho about our products and services, but if they see other people making positive comments about them, that's very helpful.