Perhaps a compromise is to add a `NoColor` attribute value for `FillColor`, something like
```haskell
data SomeColor = forall c. Color c => SomeColor c | NoColor
```
 and use this instead of `transparent`. Then the SVG backend could handle `NoColor` with `fcA transparent` and other backends would do whatever they need to, which for existing backends is nothing. I would have to put some more thought into whether this would work.