Hmm, it's a good question for which I can find no ready answer, the 1864 dress regulations only go into detail for the officers. I believe that in theory it was meant to be white chevrons up to and including sergeants, gold for colour sergeants etc.
That said, there are illustrations of sergeants with gold. The relevant Osprey, The British Army on Campaign 1816-1902 (3) 1856-1881 has plates depicting a sergeant of the 71st (HLI) at Ambela in 1863 and a lance corporal of the Royal Engineers, both wearing gold stripes on campaign frocks. Seems that regimental dress regulations and possibly availability were the determining factor.