Tom has been talking about this for a while ... that we don't have enough basic information about color changes in roasting. Understanding the stages of the roast is fundamental to successful roasting; it is hard to emphasize it enough. He shot a macro-image video under very strong light to show color changes during the roast process. He emphasizes the development of each stage (in both air and drum roasts) and what you can expect to see and hear. The end of the roast is crucial; in the 15 to 30 seconds from first crack moving toward second crack, your roast very quickly goes from City+ to Full City+ (and beyond!). Make sure to pull up a stool and pay attention to the end of the roast not matter what method you use.
I've added this video to our Visual Guide to Roast page as a permanent fixture. Hope you find it useful. - Maria