Stick, I think you're looking at things rather backwards here. UE3 doesn't force developers to use a grey color palette, the developers who use UE3 choose to use a grey color palette.
Think about who has used UE3 the most over the past two years... Western developers. Those guys have been in love with the color grey for the past half decade. Now that UE3 is being used occasionally by Japanese developers (including SE), we'll see a bit of diversity in styles and color palettes. It's not a limitation of the engine, just a preference by most of the developers who have used it up to this point. Remember that Western devs are much more inclined to use middleware to help in development of a game than are their Japanese counterparts.
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