Impossible to say what percentage were dark gray. There are some things that just can't be determined from what information is available. Another factor to consider is the material used. In 1864 trousers could have been made of jean wool, English Army cloth or satinet. We know what color the EAC was but supplies of jean wool and satinet were obtained locally and could have been any shade of gray or just close to gray. The dyes for this material were usually organic and could change color as they aged. If you are creating a uniform sooner or later you will have to make a decision based on what the rest of the army was wearing and use that information to determine what material and color you want to use.