At the risk of being extremely unhelpful, I'd go so far as to say that the whole 'light, medium, dark' thing is pretty much meaningless. For the past 5 years or so, I'd also say that the colour of the coffee is also not a particularly good indicator of roast development. It's a difficult subject to have a concise answer for and of course our learning is continuously evolving.
A few quick statements which we currently have a reasonable degree of confidence in:
We measure colour but (over many thousands of roasts and hundreds of different beans), the 'number' we get is only really useful for us internally as a measure of consistency between batches, so we only use it as a quality control measure.
We regularly see lighter coloured coffee that is more 'developed' (i.e. more soluble and with sensory aspects that are indicative of more development (i.e. less acidity, more sweetness, more bitterness etc).
Colour tells us next to nothing about how evenly the coffee has been roasted, all sampling methods effectively take an 'average' colour reading of the roasted coffee material.
Changes in processing methods (particularly in the last 3-5 years) has made a HUGE difference to colour. Agtron is a scale of brown essentially but we see that a lot of coffee these days has a strong orange or red hue to it. As far as we can tell, this is generally related to the processing method - but also new, or rare varietals.
My own take, is that you really have to find a roaster (or some roasters) that tend to roast coffee how you like to drink it. For the customer, the main concern should be the roasters ability to roast consistently - whilst taking into account things like green coffee aging, seasonal variations in ambient conditions etc etc.
We approach the roast development issue in a fairly simple way, aiming to roast the coffee as little as possible whilst meeting some basic conditions:
Coffee is soluble enough that someone with a half-decent grinder can make a good brew without too much difficulty.
Espresso testing in our coffee shop, usually over a few days.
Acidity and sweetness (as far as both are present in the coffee) are balanced as well as they can be.
Agreement within the team that the coffee tastes good and indications are that we can't improve the roast.
We achieve these things in a number of ways, including:
- Standardised brew testing where we measure TDS and use calibrated grinders etc etc. Coffee needs to sit within a reference range before passing this stage.
- DTR% is used initially to get us in the right ball park for a new coffee, usually aiming for 19-20%. There are a load of issues and misunderstandings with this stuff too, but that's another subject completely. This is just a starting point for a new bean, we will almost always develop multiple test roasts in the early stages.
- A reference range for ROR at the end of the roast.
Happy to expand on any of the above if anyone is interested, and there is probably more that I've not mentioned but I've got a newborn baby here, so I'm actually quite impressed with myself in being able to string a few sentences together. 😊