nonicman, many brewerys here have different recipes for the cask and bottle/nitro versions.
This has gone on for many years now, and some have great variations. This may be not only colour, but drastically different flavour and even ABV!
Some good examples:
Batemans Combined Harvest: Bottle 5.7%, Cask 5.9%
Black Sheep Riggwelter: Bottle 4.9%, Cask 4.7%
Fullers London Pride: Bottle 4.7%, Cask 4.1%
Fullers ESB: Bottle 5.9%, Cask 5.5%
Greene King Abbot Ale: Bottle 5%, Cask 4.9%
Morland Original: Bottle 4%, Cask 3.5%
Ruddles County: Bottle 4.7%, Cask 4.3%
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