I don't think this is a particularly good comparison since you're trying to compare two very different scenes with very different lighting and times of day. The Forest of Tethir in ToB is meant to be a mysterious forest which is why it's lit that way which is a lot different to the BG3 screenshots which is instead set in a "mundane" area at midday. A better comparison would be to a lot of other daytime areas in BG1 such as Lion's Way or Cloakwood Forest which aren't really that much different in terms of artstyle (with the major exception of the much more advanced 3d graphics and lighting of bg3). You can even compare it to a lot of similar daytime locations in BG2 such as the Windspear Hills or the Temple District and see that there's not nearly as big of a difference as you're making it out to be.

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The original BG games were never particularly dark and somber visually (although there were a couple areas that were). BG1 in particular is often very colourful and bright.


BG2 was a bit more grey due to its heavy use of pre-rendered cg backgrounds, which resulted in a slightly washed out look which was common in most games that used pre-rendered backgrounds such as basically every other IE game, Arcanum, ToEE, Resident Evil 1, and a ton of JRPGs of the era. That being said it still had quite a lot of colour.

You're also ignoring a lot of BG3 locations that are not nearly as bright and have different colour palettes. These areas include the swamp, the goblin camp, and most interiors in general.
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The blood puddles are a little over-the-top, but they mainly exist as a gameplay mechanic since blood puddles are surfaces. It's not that big of a deal though since most CRPGs do have pretty exaggerated gore. BG1/2 had enemies exploding into chunks on crits, meanwhile the Pathfinder CRPGs have enemies exploding into ribcages.