Color::invert swaps channels incorrectly

### What happened?
Color::invert currently computes:

```rust
r = 1.0 - r
b = 1.0 - g
g = 1.0 - b
```

This cross-dependency causes incorrect inversion.

### Expected behavior
Each RGB channel should invert independently:

```rust
r = 1.0 - r
g = 1.0 - g
b = 1.0 - b
```

### Scope
Minimal fix in color.rs only.