Be careful with dress codes that require employees to purchase the employer's clothes, or a specific design or brand. In states where employers are required to pay for uniforms, the employer may need to pay when it requires employees to wear a specific brand. The Gap found this out the hard way, see http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2005/01/28/BUG46B1PJQ1.DTL&type=business and now may have to reimburse employees for the clothes they purchased.
I used to work in retail apparel. Most companies give employees substantial discounts, which means that they usually wear clothes from that store.
Of course, you may have employees that don't fit the target market of the line (too old, too young, too large), so they wouldn't necessarily encourage buyers by wearing the clothes.
You can have a dress code such as no jeans, no sneakers, collared shirts, etc. or ask that employees wear certain colors, such as black bottoms and white tops, without inidicating a specific clothing brand.
KMoore1 is right, if you ask employees to wear a specific tee-shirt, etc., it may be considered a uniform. My company gave out promotional shirts for employees to wear during special sales.