All the stones that I engrave have 3 coats of black paint within the engraving and a top coat of clear, acryllic paint as and added sealer. I do recommend that every 2 years, a clear coat of spray paint is applied on top of the stone.
If you see any moss or mold growing onto the stone, I suggest that you set the stone in bleach. And if needed, use sandpaper to gently rub off the moss or mold. Then apply the paint. Also, when you place the stone back into your garden, make sure to place the stone on top of light gravel or wood or anything to keep the stone directly off the ground. That way the mold won't get back onto the stone.
It's very easy to keep these stones looking great forever.