翻译：I have a colorful shirt.
Here "one" is used to emphasize the number of colorful shirts. On the other hand, "I have a colorful shirt" is used to convey the mere fact that I have a shirt that is colorful.
Eg. Can you wear something pretty for the picnic? Hmm, I think I have a colorful shirt that I can wear.
Did you pack your clothes for this trip already? Yes, I have one colorful shirt and two black ones.