A red shirt primarily reflects red wavelengths of light. When light hits the shirt, the fabric absorbs most other colors in the spectrum and reflects red back to our eyes.
This happens because the dye used to create the red color absorbs different wavelengths. For example, it absorbs blue and green light, which means those colors are not seen. Instead, the red wavelengths bounce off the fabric, making the shirt appear red to us.
In addition to red, a small amount of other colors may be reflected depending on the specific shades and materials involved. However, the defining characteristic remains the predominance of red reflections, resulting in the vibrant hue we associate with a red shirt.