More stuff about beer colour that I posted as comments. So they don't get lost, I'm putting them here. Purely for my convenience. Don't feel obliged to read any further.
Amber Ales, like Brown Ales, disappeared just after 1800. The names popped up again later (Amber beating Brown by a couple of decades) with totally different meanings.
I'll be honest here: I've no idea what an Amber Ale was. The name is used fairly erratically, usually referring to bottled beer. I've also no idea what the colour was. I could guess. But we all know how dangerous that is.
What's meant by amber as a colour? I'd describe Bitter as amber in colour. And one Amber Ale I did see, Newcastle Amber, was paler than most Bitter.
What I do know, is that Barclay Perkins had different coloured Milds in the 1920's and 1930's. Sometimes they were sold "as brewed" others darkened with caramel. Before colouring they were about 40 EBC, after 90. Their Pale Ales of the period were 21 to 26.
I've seen odd examples of amber malt used in Milds 1890 - 1910. And in Stock Ales. Because, of course, they went dark too. What you see in the logs are the use of fairly small amounts of amber and brown in the beginning. And some dark sugars. The amounts increase until, by around WW I, most Stock and Mild Ales are pretty dark.
In the breweries I've looked at, almost all the colour comes from sugar. About the only time they used much dark malt was during the wars, when they couldn't get the sugar.
Before the late 1800's, getting any colour other than black consistently would have been just about impossible. In Britain, at least. When caramel was allowed, colour adjustment became practical.
Even in the 1930's, Barclay Perkins struggled to hit their target colours. They had to adjust the colour of just about everything. OK, you'd obviously aim low in the brewing because you know you can only adjust up. But I've seen plenty of logs with "too dark" scribbled on them as well.
The why is an even tricker question than the when. This is my crazy guess which has only minor basis in documented fact.
1890-ish is when, according to one source, pubs in London were going from serving in opaque tankards to glass. So drinkers could notice more subtle differences in colour. The way beers were brewed at the time, from 100% pale malt, would have made the stronger beers a bit darker. Punters associated darker with stronger, so brewers made their beers artificially darker so they looked more attractive. Continue this process for 30 years and you've gone from pale to dark.
Just a total guess. Don't quote me.
Beer colour is a fascinating subject, but frustratingly difficult to research. Without a time machine, I fear much will never be certain.
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