Unfortunately, the default potion colours are used by adding colour filters to the potion texture you found. The default potion colours are hardcoded into the game. The only way to change the way the colour filter works is to put an NBT tag on the potion item with a number representing the colour of the potion. The game automatically changes the colour of the texture to match the number you select.

This tag is called CustomPotionColor, and the formula to find the correct value is (R << 16) + (G << 8) + B.

Sample:

give @p minecraft:potion{CustomPotionColor:16777215}

The only way to make a colour as the default colour is to set up a system that can detect a potion in your inventory and automatically replace it.

this unfortunately is impossible...

...because of the way the textures are handled. like ExpertCoder said, you can create your own custom in-game colors, likewise with leather armor. because of that, Minecraft takes the potion textures (which have a transparent gray texture by default) and applies an "overlay" of sorts to give it the colors it wants. since those colors are coded into the game itself, and not accessible by the player (in a way that doesn't class as a mod), you can't directly edit it.