They didn't.
The group who wore the gray uniforms were the Confederates from the south. The Union soldiers of the north wore the blue uniforms.
In the US Civil War, the soldiers of the north wore a dark blue uniform, and the soldiers of the south wore gray uniforms. The war is still remembered in terms of blue vs. gray.
Confederate soilders wore gray or yellowish-brown uniforms.
The Confederates (south) wore gray and a color called Butternut, and the Union (north) wore a blue-gray more blue then gray type of uniform.
Gray. Confederate uniforms were gray with red piping.
They wore gray uniforms
In the US Civil War, the Union (North) Army usually wore blue. The South wore Grey but often they didn't have official uniforms.
they wore little gray and the shirts had buttons on.
The Confederates, unlike the Union, didn't really have a specific color to their uniforms. The colors ranged from tan, to gray, to a lighter blue. This was due to the fact that the South did not have manufacturing corporations where they could have their uniforms made. The North's uniforms where very specifically colored dark blue.
The confederates wore grey uniforms. Though shortage of uniforms often meant that some confederate soldiers wore clothes from home. Union uniforms were a dark blue.
The Union uniform was blue, the Confederacy uniform was gray. Other than the color, the clothing was quite simple and ordinary. Uniforms were not elaborate.
Confederate uniforms, gray wool uniforms