ENH/BUG: color cannot be applied to line subplots
Specifying `subplots=True` in line and kde plot draws all lines in black, and may not accept colors and styles keyword. Summarized a table to compare current / expected (fixed) behaviour.

Sample data:

```
df = pd.DataFrame(np.random.randn(10, 3))
```

| Command | Current Behavior | Expected Behavior |
| --- | --- | --- |
| `df.plot(subplots=True)` | All 3 lines are in black (OK) | 3 lines are in default color cycle. |
| `df.plot(style='r', subplots=True)` | All 3 lines are in red (OK) | No change |
| `df.plot(color='r', subplots=True)` | All 3 lines are in red (OK) | No change |
| `df.plot(color='red', subplots=True)` | All 3 lines are in red (OK) (thanks to #10387) | No change |
| `df.plot(color=['r', 'g', 'b'], subplots=True)` | ValueError: to_rgba: Invalid rgba arg "['r', 'g', 'b']" (NG) | 3 lines are drawn in red, green, blue |
| `df.plot(style=['r', 'g', 'b'], subplots=True)` | 3 lines are drawn in red, green, blue (OK) | No change |
