From green to yellow to fithy shades of brown.
Color change is a good way to follow the roast. It is more solid than bean temperature reading that is influenced by other things than what is going on inside the beans (see sensitivity to airflow).
It is difficult to make establish “Now it’s yellow” and note the time … or “now its the dark brown color where I dump the roast”. The color changes (CC) are gradual and the beans do not change evenly. You just have to find your own way of doing it. Like; when 80% of the beans are no longer green, I call it yellow.
Also – what a color look like depends on the lighting.
Here I have two different lights on my roaster. The photos are taken 10 seconds apart. The first light gave a more yellow color – than the new ligth (the one with a a circle of light around a magnifying glass).
Read more about Yellow point at http://coffeenavigated.net/roasting-coffee/#yellow
Listen to podcast episode 3 and 7 where roasters talk about how they use the bean color during the roast http://coffeenavigated.net/podcast-roasting-navigated/