Progress Bar

How do you change the progress bar color using GDscript?

You can use a shader for that, or you can also use the ColorFrame node, which will be released soon, but is already avaible in non official releases.

The ColorFrame node can be used as a Panel, but with a Color that can be easily modified by code. :)

You can either use a TextureProgress and supply your own textures, or a ProgressBar with a custom StyleBoxFlat (which allows for any RGBA color).