I concur with Carlos, khaki is the most likely answer.
That being said, I myself have always fielded them in that distinctive very dark -- as Carlos says, almost black -- Rifle Green. There is a photo of the 3rd battalion in 1882 Egypt, wearing their Rifle Green uniforms. However it was not the 3rd but the 2nd battalion which served in the Second Afghan War.
Over many years of interest in the Second Afghan War, I don't think I've ever seen a contemporary drawing or photo depicting the 60th Rifles during the campaign. They earned Battle Honours for Ahmed Khel, the Kabul to Kandahar march, and the battle of Kandahar, so it stands to reason that somewhere there must be at least one such contempary illustration, or at least a written description. Your next step might be to post your question over on the Victorian Wars Forum, which has an area dedicated to the First and Second Afghan Wars. Someone there might have a better informed, maybe even definitive, answer.
Here's the pic from 1882 Egypt I referred to above: