I tend to stick to 10-15 minute decoction boils and only do double decoctions, rarely do I go all the way to a third decoction (not that I have a problem with them, I just dont do triple decoctions).
So far in my experiences with decoctions, I have a rough estimate for color increase.
For light beers in the 5-10 SRM range (starting) a single 10 minute decoction usually gives me approximately 1.5 to 2 SRM increase. AKA a pale straw colored beer will move in the direction of yellow/gold. Yellow/gold beer will move close to amber.
Darker beers in the 13-20 SRM starting range it seems like the color change increases quicker. Not by much, but a single decoction I get closer to a 2.5-3 SRM increase for a 10 minute decoction. My sensory notes I think closely match what is generally observed by others for color increases.
I don't think the color increase is linear though since you mix the decoction back into the main mash, rest, then pull another decoction, so you are maybe boiling some of the previously decocted thick mash. Theres so many things that come into play with the color side and maillard reactions in the pot when you are boiling the decoction (surface area exposed to heat, intensity of the heat, what type of pot it is in aluminum/stainless steel/non-stick).
I use a non-stick 1 gallon pot for my decoction and my electric stove. It has a relatively tall contruction so theres not a ton of surface area on the bottom. But since it is non-stick and I use an electric stove I greatly decrease my chances of scorching the mash, as well the temperature is usually very very steady, all I have to do is just stir the hell out of it during the boil.
TLDR: You will hit your color range for most of the 'darker' bock styles, I don't think that you'll fly past the color range though either.
