Why's is it so difficult to pin down? There are a number of reasons.
- First, and fairly obvious, is the fact that only in the 1920's did they begin to record the beer colour on logs. (And even then, only on some.) That means you have to try and work it out yourself. Not always as easy as it sounds.
- Which leads onto the second reason. Being unspecific about the kind of sugar being used. The entries often just say "sugar". Even though the invert sugars still used today - No. 1, No. 2, No. 3 - were already around. When no dark malts are used (mostly the case) it just leaves you guessing as to what effect the sugar might have had on colour.
- Breweries adjusted the colour using caramel, but didn't always write it in the logs. Especially if the adjustment took place after the boil.
What's the point of all this? Well take a look at this Barclay Perkins log from 1886:
Do you see? Towards the top in the middle: "12 Garton's No. 3 Sacc." That's dark No. 3 brewing sugar. Given that it makes up 25% of the grist, I think we can assume the finished beer must have been dark.
So when did Mild turn dark? Well, at least one Mild was dark by 1886. That's progress. It's the first real evidence I have from before 1900. I wonder if I can push the date back any further?