Try Crayola washable markers. They come in lots of colors, affordable and wash out easily. I'm sure you'll find a color that will show up. I have used 2 and 3 different colors on 1 quilt before so it would show up well.
A couple weeks ago, I tested all the colors, using the Crayola washable markers, trying different ways to wash them out. I tried dabbing at them with a wet cloth, immersing the cloth in cool water and washing in the machine with and without laundry detergent. I used a separate muslin cloth for each method. The colors, including black, came out totally except for the dabbing method. Somehow, instead of disappearing completely, the colors would migrate to other areas of the cloth.