Boy, this overlay is devilishly hard to work with! For one thing, I do not believe that the scheme is simple RGB. Rather, it seems to be sort of like this:
* --
* -- (Green)
*
*
* -- (Blue)
* --
* --
* (Red)
* --
The colors marked "* --" are the active colors in the memory values (nonzero values). What's more, a "light blue" seems virtually impossible to create. It's like there's a huge jump in color saturation with no middle ground.
Here's three options that seem better to me than the rest right now:
No overlay
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Light Blue (still a bit too dark for my tastes)
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Light Purple
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My fave is the light purple. True, it doesn't match with the blue overlay of the indoor environments, but it still signals that the player should expect darker environments. It would actually make sense for the indoor environments to be darker than the outdoor environments at night. That leaves the outdoor land bridges as standouts -- there's only a few of those, so is there an indoor overlay function I can tinker with?
And then there's the original palette with the overlay Chrono'99 originally had, which is looking superior to the more recent attempts IMHO:
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I'm tempted to go back to the original palette and maybe tweak the overlay so that it's just a bit lighter.