single column DF plot, color problem
```
import pylab
import pandas.util.testing as put
df = put.makeTimeDataFrame()
pylab.figure()
df[ ['A'] ].plot(color = ['r'])
pylab.savefig('C:/tmp.png')
```

will give an error

```
raise ValueError('to_rgba: Invalid rgba arg "%s"\n%s' % (str(arg), exc))
ValueError: to_rgba: Invalid rgba arg "['r']"
```

All you need to do is to de-list the color parameter

```
df[ ['A'] ].plot(color = 'r')    
```

No problem if df is more than one columns.

```
df.plot(color = ['r','k'])
```
