为什么我不能为UINavigationBar设置一个自定义的颜色?

I want to change the color on the navigation bar and the documentation states not to attempt to change the 'alpha' value. So, how can I set a color that is not the SDK defined values (e.g., [UIColor redColor], brownColor, etc.) if all the constructors provide an alpha value? I tried getting the current alpha value but that did not help. When I use a custom color, it causes the navigation bar to look like an x-ray (a scraggy, black-on-white).I want to change the color on the navigation ba