What color t-shirt is worn with the ACU uniform?

The ACU (Army Combat Uniform) is designed for functionality in a variety of environments, and the specific color of the t-shirt worn underneath the ACU is tan or sand. This color is chosen to maintain a uniform appearance that blends with the camouflage pattern of the ACU while also being practical for wear in outdoor conditions. The tan or sand color provides a neutral base that can complement the various shades present in the uniform, enhancing cohesion and ensuring that the soldier's attire remains professional and aligned with military standards.

Other colors, while they may be used in different contexts or military branches, do not align with the specified requirements for the ACU. For instance, white shirts, often associated with dress uniforms, would stand out and not serve the operational purposes effectively. Black and gray t-shirts do not provide the same tactical suitability or matching aesthetic that tan or sand does when paired with the ACU. Therefore, the correct answer focuses on ensuring functionality and uniformity within the military dress code.