Now days it's pretty common for progress or percentage bars to use a gradient from start to finish.
The problem I always encountered was that I wanted the gradient itself to represent progress.
Dark for low values (i.e. 10%, and light for higher values (i.e. 80%).
Having said that, here is my approach to solving the problem. If you want to see a starker contrast, change $rose/$peach/$teal/$lime to more contrasting colors (black/white for example)