The paint color may not be "that desirable" but isn't it original and beauty is in the eye of the buyer. I looked for the web site by Spooky Tom or is it Spookie Tom, where he has pictures of all the original paint colors for SOHC/4s by year with frame and motor serial numbers, but couldn't find it in a quick search. I have it bookmarked on my computer at work.
I'd say the bike looks great. The question, with 50K miles, is how good was it maintained mechanically? If very well maintained, 50K isn't so bad. But if not . . . . But, if the exterior is any indication of the interior, it may be good shape. Can you do a compression check, etc.?
I see a lot of post here about buying good bikes (like this one may be) for $700 but in my reality those are very few and far between. I wonder if those who were lucky enough to score a good bike at that price were just lucky and boasting a bit. I searched for an earlier 750 every day for at least 6 months in Cincinnati, Columbus, Indianapolis, Dayton and Lexington and the best I could find cost me $1,400. And by all appearances, it's not in as good a shape as this one. And I've put $300 to $400 into it (not including my time) already to get it safe and roadworthy.
I also picked up a '75 550K with 7K miles and very clean and all original a year ago. I paid a bit over $2K and aside from tires have not spent another nickel on it. It's a beaut and I love it. Did I pay a bit of a premium? Maybe. But if I were expecting to find this bike for under $1000 I'd either still be looking or have won the motorcycle lottery.