Hi, its my first blog so i am going to teach you something easy guys.
I love how minecraft let you express your creativity and how you can personalize your game by texturepacks and mods.
So how you can change loading screen color with optifine resource pack?
First, in your resource pack you should create new folder, called "optifine". the path should looks like:
assets>minecraft>optifine
Then in "optifine" folder you should create text document called "color.properties"
(lets pretend that your favourite color is purple so you want to change loading screen color to purple)

In this document write or just copy from there: WARNING! If you copy, you must copy "line by line". All lines MUST be in their place like below! "#" symbol means "comment" so all things after this symbol are not important and you dont have to copy them.

###############################################################################
# Resource loading screen
###############################################################################
# Background color
screen.loading=a26b8f
# Loading bar background color
screen.loading.bar=6ba29a
# Loading bar outline
screen.loading.outline=fdfcfc
# Loading bar foreground color
screen.loading.progress=4a5176

###############################################################################



the "6ba29a" means colors, you can check what your favourite color is named on this site: https://www.colorhexa.com/6ba29a

Enjoy! If you have any problems, write in the comments.