Hi all. Im new here but was reading thru posting about different types of concrete used.  I have made more stepping stones than I have stain glass items.  I was told a while back that concrete can be tinted using latex paint.  I made several stones tinting them with a grass green acrylic paint (the craft type)  I also did one with exterior latex paint.  They both came out fine.  The green came out as a med pastel green.  You could experiment making a coaster size item
to test amt of paint color needed to get the color needed and adjust amount according to size of stone being made.
Bear in mind that the paint needs to be accounted for in the amount of liquid used to mix the concrete.  I also sealed my stones using Thompsons multipurpose concrete sealer.  I set my stones in the ground, digging out enough so they were somewhat level with the ground.  My husband had to mow over.  He wouldn't go around.  My stones were in the ground around 8 years before one of them lost a piece of glass.  The stones I tinted with the paint I can't tell you about because we sold the house about 2 years after I set them in the ground.  Until that time they looked the same as the day I set them there.  I hope this helps.  Oh, I also made my stones using morter mix for the first 1/2" then some concrete. a piece of chicken wire for stability, topping of with concrete.