How to make white ready-roll icing brown?

That wouldn't work too well. Add some red and green (or Orange and very little black) food coloring- sparingly add black to darken. But red and green should be able to darken to a point- you want to get it to a deep shade or orange before you add the black. If all else, just make it lime green or pink- if you has the skills, you can make the colors swirls without mixing so much to give something of a tie dye effect.