Why does it matter what colour the planes are painted? The average First Choice, Thomson, Thomas Cook, MyTravel passenger wouldn't know and probably don't even care what it says on the tin!
Perhaps the FC/TUI management felt that there were more important things to resolve following the merger than what colour the planes are painted! Shame the TCX/MYT management didn't see it that way.
It's a mixed message from a marketing point of view, and regardless of how dumb you apprently think Thomson passengers are, there are still people looking for their Thomson flight being confused by a First Choice coloured plane on the gate! It's not ideal....