If you're going for visibility, the neon red ones. If you have no concern for crazy coloring schemes, could maybe pick up a neon green or orange or something in the future. Up to you, of course, how you want to run it and what colors you're going for.
That all said, I was worried the green coloring of the canopy on the 550X was going to blend with all the foliage around here and that I'd have a hard time with it. Has turned out not to be an issue. And the few times it is, easy enough to climb it or fly it in a little closer.
Was also worried about keeping orientation at distance due to my experiences with the sim and so easily losing it in there (even at high resolution settings). In real life I've never, so far, had a problem of flying too far away and losing orientation.
You may be trying to solve a problem that doesn't really exist.