Did something similar a few years back for the tar expanion joints on a concrete airfield ramp. Bristol Board from the arts & craft store makes a very nice concrete - find one off-white to slightly gray.
You will need a syringe and an appropriately sized needle. At the craft store, get some black or dark gray fabric paint. I think the stuff I used was a puff paint - used for making decorative dots on fabric.
Draw some of the paint up un the syringe and apply it in lines on the base. The procedure is similar to caulking a window with a caulking gun. Your natural hand waver will give it a slightly wavy line that you want. Because it is a stiff paint it will retain a raised appearance - not be a line painted on the base.
It takes a bit of practice.
DO NOT TOUCH it until it is dry.
You may be able to get a similar effect using craft acrylic paint. This will not be as stiff and may not stand as tall. Again - let it dry.