Personally I like bright colors on bikes. I had a yellow SR500, another SR with flames painted on the tank, a red/white/blue Honda Nighthawk S, and like the bright Ninja... but that's me.
There is some goofy fix with a lot of riders on black bikes. Personally I dislike them in general. Black shows every swirl and defect in the paint, as do all dark colors, it's just the worst. The bright colors like yellow, orange, and red won't show swirls and light scratches in paint.
I will mention I had a friend who didn't particularly like yellow bikes, but ended up with a yellow 750 Nighthawk and loved the look with the black engine.
It's all about what YOU like. Go check it out and if YOU like it in yellow that is good. If not, walk away. My friend sold his 750 and one of the guys that looked at it simply walked away because he wanted red and red only. I did the same when I was looking at Nighthawk Ss again (sold mine when I got laid off years ago). I would only take the 1986 red/white/blue model or one of the Zephyr 550s. I could find other Nighthawk Ss, but not the r/w/b model. The Zephyr showed up first so I got it.
Now on that specific bike, if he has papers showing service records and it's as nice as it looks, it seems to be a great buy. You couldn't get a 250 for that price. Look at mufflers, shifter brake lever tip, foot pegs, fork ends, and brake calipers for scratches from going down. If all is clean or if evidence of only tip overs, that is probably a good choice - provided you like the yellow.