Just as visible panty lines are a no-no, visible bra lines are as well. Stick with a color that is close to your skin tone under light fabrics and sheer shirts. Do not wear lacy or heavily embellished bras with low-cut tops or blouses in a professional or otherwise conservative environment, as the design of your bra might be visible.
A t-shirt bra works best in any of the above situations. Its seamless style and comfortable feel will make your clothing look smoother on your silhouette.