FormattedText Span foregroundColor and backgroundColor cannot be set using known color

For example:

``` XML
<Label>
  <Label.formattedText>
      <FormattedString>
          <FormattedString.spans>
             <Span text="Test" foregroundColor="red" />
          </FormattedString.spans>
      </FormattedString>
  </Label.formattedText>
</Label>
```
