White is the most visible / brightest colour.
That said, the red and green might stand out better on a lighter sky . . . I found my dual white wasn't visible more than about 700m on a light grey cloud sky, low to the horizon.
I also have a public safety series ARCII in the red / white option, not bad.
Just how far away are you flying for this to be a problem ?
I can see my ARCII white on any sky, out to about a km.
Flying past that is just not a good idea for real VLOS / airspace awareness.
If you need something brighter, the ARC XL is the best I know of (apart from bulky, heavy, flight time draining spotlights).
The XL is 1000 lumen, vs the ARCII 600 lumen, so really a lot brighter.