How to reproduce faux tortoise shell material I'd like to mimic this faux tortoise shell material: 



Ideally I need the material to be like a thin layer of plastic (probably less than 2mm).

I tried using clear epoxy resin + colors, and I almost got satisfactory results (at least for the first try):



But by the time the resin was curing, the colors were all mixed together, and the final result was a single color. So I guess I need to find another way of doing it.
Any suggestions? I was thinking about using acrylic colors on a thin sheet of plastic, and then using clear resin over it when the colors are dry.
I've recently viewed a YouTube'r creating pen blanks with two different colored resins and pine cones. She placed a timer in the camera view and after mixing the two parts of the respective resins waited for a specific time before pouring the resins into the mold.

I did not pay sufficient attention to the elapsed time and it would depend on the resin being used. I've seen resins that gel up in three or four minutes and another product that has forty-five minute working time.

You may be able to use your existing resins if you are able to determine the edge of the gel period.