The human eye can also produce the same "floating" effect with colors, as I've often noticed the different colors of red and blue, especially in LEDs, do not seem to be lined up on the same level in many lightbars, especially from a distance, and it does look very odd. However, it seems to be more apparent in some of the various bars on the market rather than others, probably because of number of diodes in the LEDs, their positioning, etc. If you do have the option with your lighting color presentation, mixing red and blue is the ideal way to go, as it obviously covers the classic day/night argument. However, blue's visibility has progressed these days to the point where it is very effective when used by itself.
Back in the my LE days in Arkansas in the early 80s when we still used Visibeams, we had all blue on our PD cars. Being from Louisiana, where red/blue was often mixed, I decided to change to red/blue on my unit. I borrowed a red dome from the FD guys and used it for a couple of days until I found out from the chief that the state code dictated red was strictly for fire/EMS use, so I had to remove it and return to all blue. The all blue was very dim and hard to see in the daytime, so for kicks, I borrowed a clear dome from the FD to try out. Since I didn't have any blue bulbs to use under it, I had clear on one side of the bar and blue on the other. That lasted for about half of one shift as it looked like the bar was missing the colored dome entirely. I went back to all blue domes, but soon put 100 watt 4509 aircraft landing bulbs in the bar to make it brighter, as those 35 watt 4416s just didn't cut it. Of course, it slowed the rotation down so much, I had to alternate the bulbs and have one of each per end. I eventually changed to the 50 watt 4464, which worked out best. Coincidentallly, Arkansas eventually changed their color codes and lets red and blue be mixed for LE use now.