Fixed issue with not being able to select color white

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.

#### 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