Fixed issue with not being able to select color white

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31188

Because the white value is uppercase (#FFFFFF), the javascript trying to set the value (in lower case: #ffffff) fails.
The value should be lowercase and this PR fixes that.

Also fixed the issue with uppercase rgb colors in the custom list.
So this now works too:
```
<field name="mycolor" type="color" default="none" control="simple" label="Simple: Custom Color List"
    colors="none,#FFFFFF,#000000,#FF0000,#00FF00,#0000FF,#00FFFF,#FF00FF,#FFFF00" />
```

#### Testing instructions
Test an item that uses the field type color (with control="simple") and try to set the color to white. before this fix it will safe as none (transparent) and after it should work.

For more info about this field type, see: https://github.com/joomla/joomla-cms/pull/807