How to fake this concrete effect with paint?
You can use a natural sponge to make concrete like shadings.  Use a light touch and dab the sponge.  Turn the sponge often so you don't get a regular pattern. Practice first on some cardboard. You can start with either your darker shade or the lighter, see which you like the best. Go back over what you have done with a dry brush or rag to soften your work while it is still wet. Remember acrylic paint dries fast and darker.