I love the look of stained concrete Suzanne ! It turns out beautiful and can even look like
marble if you layer the color. Since your base colors are neutrals, gray and black, you can get away with just about anything . Blue, darker gray, or even maybe some green for a pop of color. I suggest painting a few large pieces of cardboard a color that you like and laying them down on the patio and living with them for a little while. You might be surprised at what grows on you or looks good. Good luck and happy painting !