Nude colored bras are a great fall-back if you are unsure what color bra to wear under a lightly colored shirt. They come in a variety of styles and cuts and help to create a smooth seamless shape. Nude is the surefire way to avoid wearing the wrong color bra under your shirt or blouse.
When choosing a bra to wear under a light-colored or sheer shirt, pick a bra close to your skin tone. That will help the bra show up less than if you picked pure white.