This tutorial might be helpful: https://www.youtube.com/watch?v=k8bmMU91e_I
You can paint them as Lauren suggested or you can actually make them from concrete. Here's a project from another Hometalk member if you'd like that option. Wishing you the best.
Keeping this idea!
try painting a thin layer or plaster on them and then painting them grey.
while waiting for an answer, I did plaster! Now paint gray, or spray it with cement paint???
Cement spray paint!
thank-you. Here I come Home Depot 😃