How do i animate my health bar so when i take damage it lowers and changes colour.

This is a picture of my health bar ~ Screenshot - 2c837edb3cc570ebe1f855aa94c6014f - Gyazo

Should i make 50 versions of it slowly reducing the green bar? I know there are many solutions for this, but my healthbar has a different shape, im not sure how to work around this.

The technique used for the circular progress bar will work with your shape:

You can handle the color either by using a version of an alpha cutoff shader that supports an overlay color, or you can make several textures (complete fills) in different colors and swapping the textures as needed.