VMobject cannot achieve gradient effect

### Describe the bug
Before, the following code can achieve the gradient effect：

**Code**:
```
class Bg(Scene):
def construct(self):
bg = FullScreenRectangle(fill_color=["#032348","#46246d","#31580a","#852211",])
self.add(bg)
```
**Output:**

**Wrong display or Error traceback**:
But now the output is

### Additional context
Is there a way to change it to the previous effect, or is there a new way to achieve it?

Look at the VMobject and FullScreenRectangle classes in manim, particularly the fill_color handling and gradient generation. Check recent changes to color interpolation or shader code. Run the provided code to reproduce the issue, then compare with older versions to understand what changed. The goal is to restore the gradient effect when a list of colors is passed.

Written by the indexing model from the issue text.