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.
This coffee was roasted on my Probat 12 kilo so I could take advantage of the sample trier. Ignore the times, and take the temperatures as a ballpark figure.
The important thing is here is to see the transformation the coffee goes through as it roasts and what look, color, bean size and surface texture, corresponds to the degree of roast. ***(see note from home roaster George Steinert below).
Roasting is more about exceptions than rules. I have this page about bean color vs. ground coffee color that might be helpful. So get to it.
Note: The above image is not the exact same beans pictured below. The gray strips on either side of this image are a photographic 18% gray card.
This is a wet processed, Central American coffee, a accidental blend I have had sitting around. Each photo here are different coffee seeds from the batch I roasted so size and shape will vary seed to seed.
First crack is drawing near at this point. Some bean expansion is visible as the central crack in the coffee has opened slightly. The coffee releases some silverskin or chaff.
Notice the bean surface is smoother somewhat from expansion but still has darker marks in the coffee, like a finely etched pattern. The edges of the seed are still fairly hard. At this point the coffee starts giving off carbon dioxide.
Click on image for large photo, or here for multi-bean photo. Check here for the whole bean vs. ground coffee comparison photo or aMacro photos of a single City + bean.
The Vienna stage (also called Continental) to Light French stage is where you begin to find Origin Character eclipsed by Roast Character. A dark or heavy roast is at odds with buying coffee for its distinct origin qualities. Dark roast coffees tend to taste more like each other - as the differences due to distinct origins are obscured by the carbony roast flavors. Nontheless, some coffees are excellent at this stage (our Puro Scuro blend is engineered for this roast range).
Click on image for large photo, or here for multi-bean photo. Check here for the whole bean vs. ground coffee comparison photos or a macro photo of a single French roast bean.
Sugars are heavily caramelized (read as burned) and are degraded; the woody bean structure is carbonizing and the seed continues to expand and loose mass. The body of the resulting cup will be thinner/lighter as the aromatic compounds, oils, and soluble solids are being burned out of the coffee and rising up to fill your house with smoke. 474 is well beyond any roast I do on the Probat. I will go as high as 465 on a couple blends, and that's my limit.
This bean is right at the verge of fire - in fact you can actually start a fire with a large batch once you dump the coffee out of the roast drum into the cool tray. The sudden rush of oxygen might be the needed ingredient for cafe del fuego. Kids, grab your marshmallows!Hope you like 'em smokey!
***Note from George Steinert, home roaster: "I see a lot of new users of the iRoast2 struggling to make sense of roast levels. I have been using the iR2 since February. I got the best success when I added the digital thermometer with thermocouple probe (both from SM) and condensed the well-documented "degree of roast" information on the SM web site into a one page chart (below). By using the temps on the chart (as measured with the thermocouple planted in the bean mass) as a "guide" and when combining that with actual experience of hearing the cracks (when they can be heard through the fan noise), the color, the smell, and the time (when using a consistent load of green beans), I have had very predictable results. I continue to use the chart as my baseline of understanding when roasting with the RK drum which I started using in June. I reference the start and end of first crack to tell me how the roast is progressing. After a while, the aroma becomes a factor. About 60-90 seconds before first crack begins, I swear I can tell it's coming because of a characteristic aroma. Primarily I use the sounds of the crack(s) and secondarily, the time. With the iRoast2, the temp from the thermocouple gives a pretty reliable indication of where they are in the roasting process...given some flexibility regarding start and end of first crack depending on the type of beans. With the iRoast, you also have the helpful cues of color and seeing what's happening on the surface of the bean. Users should note that First Crack is not going to necessarily begin when your thermocouple hits 401 degrees F nor end when it hits 426 F but after you've done a few dozen roasts you will get the feel for how to overlay the progress of the roast onto the temperatures you are reading.
Please note that both Tom and George emphasize that temperature alone will NOT determine degree of roast. Each roaster is different and different beans roast slightly differently as well. All the information on this page is to be take together to help determine degree of roast - no one element (appearance, sound, temperature, etc) can determine degree of roast. Most importantly - TASTE THE COFFEE - and see what that tells you about how it roasted.
Having lived my entire life at or near sea level (Chicago, Boston, Ohio, Oakland), I may be chromosomally incapable of understanding the effects of high altitude or cooking or roasting. But I did want to make a note to pass on what seems to be the collective wisdom on high altitude home roasting, which is that in general you will see the roast happen more quickly, at a lower temperature. This is most true in small convection roasters, like a popper, or Fresh Roast or i-Roast, but also occurs in conduction roasting. So expect to adjust roast times accordingly. If you can control the temperature on the roaster at all, adjust target temperatures downward by 20 to 30 degrees.
Here is a representative image I took of the Agtron Roast Color Tiles, and might give you a basica idea of the color scale. There is a bit of glare on the left side though (most visibile on Agrton 45). Since this is such an approximation and the appearance depends so much on monitor calibration, etc, I am not going to put a ton of work into this ... I am working on a better method of sharing these roast colors information.