Red bras are not completely invisible under white shirts, but they can be less visible compared to other colored bras. This is because red is opposite to green on the color wheel, and when light passes through the red bra fabric, it absorbs some of the green light and reflects back the red light. Since white light contains all colors, including green, the green light that is absorbed by the red bra is not reflected back and does not reach the eye, making the red bra less visible under a white shirt.
Additionally, the opacity and thickness of the fabric of the bra and the shirt can also affect how visible the bra is. A thicker and more opaque fabric may conceal the color of the bra better than a thinner and more transparent fabric. The fit and style of the bra and shirt can also play a role, as a well-fitted and supportive bra can help to smooth out any lines or bumps that might make the bra more visible under the shirt