I suggest that you pick three or four colors from a paint deck; one as a mortar color and two or three as colors for your stone. Water them down to make a very thin paint. These thinned-down water-based paints will be used in the fist step to recolor the stone. You can intermix some c if these to increase the number of colors at your disposal. Use one or two or even three colors on each stone, blending the colors where they overlap.

Next, water down a portion of the new color for the mortar. This time you’ll be making a thin-as-water mix that you can apply over the recolored stones(after they are altogether dry). Have a rag handy to pat off the mix as necessary. The goal is to add a layer of translucent, dark “weathering” to the stone so that they don’t look painted.

Let me know how it works out!