Set the color to grey if one channel.

To test this PR
 * Import an image with one channel e.g. DICOM
 * The channel should now be grey instead of red (see screenshot before and after)
before
![singlechannel](https://cloud.githubusercontent.com/assets/1022396/4794518/2c8ac45a-5df5-11e4-9c60-adf7c93d8d05.png)
after
![singlechannelafter](https://cloud.githubusercontent.com/assets/1022396/4794520/2ea1ccd4-5df5-11e4-90df-0095faf6e272.png)


@mtbc: it will yes