Sky information

Sky1 is Old Night Sky
Sky2 is rain
Sky3 is thunderstorm

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Information for rain.png, thunder.png

They are used to solve a bug that occurs with Optifine:
When using a custom sky such as sky0 (assets\minecraft\optifine\colormap\sky0),
the sky becomes too saturated when it rains, snows or when there is thunder.

rain.png and thunder.png restore the classic sky color.

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greystars.png information

greystars.png is the Old Night Sky (between Beta 1.5 and 1.2.5). Stars are more grey than the Old Night Sky (pre-Beta 1.5).

If you want to use it:
delete night.png
and rename greystars.png as night.png