It might seem counterintuitive, but a red bra can actually be less visible under a white shirt than many other colours, especially if you have a medium to dark skin tone. Here’s why: 

• Skin Undertones: The red colour can blend with the undertones of your skin, making it less noticeable than a white or light-coloured bra, which often stands out starkly under white clothing.

• Shading and Blending: Red bras, especially those in deeper shades like crimson or burgundy, tend to create a shadow effect that can blend better with the skin's natural tones under white fabric.

However, there are a few further considerations:

• Skin Tone: This trick works best for individuals with medium to dark skin tones. For very light skin, a nude or skin-tone bra might still be the best option.

• Bra Style: The seamlessness and fit of the bra are important. A well-fitted, smooth red bra will be less visible than one with lace, embroidery, or thick seams.

• Fabric Thickness: The thickness of both the bra and the shirt fabric matters. Thicker fabrics will better conceal the bra colour, while thinner fabrics might still show some hint of colour. 

To ensure minimal visibility, you might want to try on a red bra under a white shirt in good lighting to see how it looks. This way, you can be sure it achieves the desired effect for your specific skin tone and shirt fabric.