Most planes have a base color of white with some secondary color. The secondary color on most is as hard if not harder to see on most planes than the N number so its not very helpful from a perspective of maitaining visual contact with a particular plane. Building a mental picture may well be possible from unique colors but seems more prone to error in my opinion due to tricks in the way our brains process information and store it in short term memory. I'd also note that colors are prone to confirmation bias. A white cessna with blue stripes (call sign blue and white cessna) looks an awful lot like a white cessna with red stripes (call sign red and white cessna) from a distance. While that is also true for N numbers (Cessa N1234 looks an awful lot like Cessna N5678 of the same type and base color), an N number is more likely to give you more pause when confirming you have N5678 in sight than distinguishing whether the cessna you have in sight is the "red and white cessna" or the "blue and white cessna." From a situational awareness perspective, as others have pointed out, the N number is a fully unique identifier. There is no question of how many planes are in the pattern. I like this. Its what I generally use personally though you can run into some issues. Not everyone is familiar with all the makes and models of airplanes but its simple enough to clarify if someone wants to know. The biggest issue really comes with the Piper lines since they are so prone to using the same name to define aircraft of widely different performance envelopes. A cherokee could be a PA28 Cherokee or the PA28-235 Cherokee-Dakota or a PA32 Cherokee 6 (with a 250 and a 300). My favorite for this is the Commanche with 4 widely different performance envelopes for the single engine PA24 lines (160, 250/260, 300, 380/400) and the PA30 Twin and its variants. Just playing devils advocate a bit with your call sign... Are you "Cherokee6 0U" or "Cherokee 60U." For this reason, I generally refer to all PA28-140 through -161 as Warriors, -180/181 as Archers, -235/236 as Dakotas and 28R's as Arrows but I realize that's just my own grouping and when I fly my friends Cherokee6 around I have to remind myself to tell people I'm a 6. This.