Information for sky1, sky2, sky3, sky4, sky5, sky6, sky7
// by Sencie https://www.planetminecraft.com/texture-pack/old-lighting-amp-old-water/

sky1 is Old Night Sky
sky2 is rain
sky3 is thunderstorm
sky4 is rain in warm biomes
sky5 is thunderstorm in warm biomes
sky6 is rain/snow in cold biomes
sky7 is rain/snow thunderstorm in cold biomes

1.21 Warm biomes
biomes=desert savanna savanna_plateau windswept_savanna jungle sparse_jungle bamboo_jungle badlands eroded_badlands wooded_badlands stony_peaks warped_forest crimson_forest basalt_deltas

1.21 Cold biomes
biomes=snowy_plains ice_spikes snowy_taiga windswept_hills windswept_gravelly_hills windswept_forest grove snowy_slopes frozen_peaks jagged_peaks frozen_river snowy_beach stony_shore cold_ocean deep_cold_ocean frozen_ocean deep_frozen_ocean

*****
Information for rain.png, thunder.png, rainwarm.png, raincold.png, thunderwarm.png and thundercold.png

These files 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, thunder.png, rainwarm.png, thunderwarm.png, raincold.png and thundercold.png restore the classic sky color.

*****
Information for greystars.png 

greystars.png is the Old Night Sky between Beta 1.5 and 1.2.5.
If you want to use it:
delete night.png
and rename greystars.png as night.png