Leica LIF: do not set Channel.Color for RGB images

The stored color for RGB images will usually (always?) be 0xffffffff, which causes the
displayed colors to be inaccurate (since the 'R' component will be shown as white).

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12214 (QA 9185).  The ```Color``` attribute on ```Channel``` should not be set at all for this file; there should be no other change in behavior.