Make color validation more forgiving

The current version throws a false positive if the style string contains `s` or `o`, which are valid marker styles and not colors.  For example:

```
style='s', color='C3'
```

should be legal, but currently throws an error.

My suggestion is to check for only the letters that are color codes.

pandas/plotting/_matplotlib/core.py

@@ -238,7 +238,7 @@ def _validate_color_args(self):
                 styles = [self.style]
             # need only a single match
             for s in styles:
-                if re.match("^[a-z]+?", s) is not None:
+                if re.match("^[bgrcmykw]+?", s) is not None:
                     raise ValueError(
                         "Cannot pass 'style' string with a color "
                         "symbol and 'color' keyword argument. Please"