That depends how far before the war you want to go. The color for infantry in 1851 regs was white/silver, and everyone else was yellow/gold. But the 1861 regs created the idea of seperate branch colors and infantry moved from white to sky blue. Since we are infantry pioneers the regs say we should wear sky blue.
Also we are not portraying the Army of the Cumberland's "Pioneer Brigade" who wore yellow badges, in line with the engineers.