Color is just one of the ways to determine degree of roast. By itself, it is of limited use. When complemented by the audible cues (first and second crack) and the aromas of the roast process, it is extremely informative . Here's a video showing the color changes that occur during roasting.
In part 1 of Stretchin' Out the Roast we looked at the effect of stretching out the time and development between 1st and 2nd crack during the roast. The greatest effect was on the perceived acidity and the type of sweetness in the cup from malt to candy, then fruit and into bittersweet-cocoa-type sweetness. In this article we look at the effect of stretching out the 1st crack itself and how that changes the sweetness, body, and acidity in the finished roast.
This article details one method to determine an ideal roast for a coffee; in four roast experiments, the time between the end of 1st crack and the beginning of 2nd crack is lengthened, and the roast stopped at the same point each time. Then by tasting and comparing the results, I arrive at some conclusions about what roast brings out the characteristics of the coffee I enjoy more. Other articles will cover the effect of stretching other segments of the roast.