I suppose you can do it with a piece of paper and watercolors. Varnish it, cut round and glue on the dial.
I did a much complex task. I have a fishbowl. And I have speedtest to check the water quality, as well as some products to fertilize the plants.
The color I wanted was pinkish and by chance one of the fertilizers I had was the one that produced the pink color, so I made a solution with that fertilizer and applied a little to one side of the test strip (with a toothpick) to that only half was dyed. I varnished the piece to stabilize it. Then cut and glued on the dial.
and perfect results thanks