Why is yellow the most popular? Isnt orange/green just as visible, or even more?
Scientifically, yellow is the most visible colour to the human eye, thus, yellow signs and warnings. Also, players tend to wear black shirts as the contrast between the colours makes the string easier to see. But I know some people who prefer orange/green to yellow.
Orange and neon green are also exceptionally visible against black shirts. I don’t know why yellow is the thing. I am more often playing yoyo for myself (and whoever happens to be around, but still myself) in whatever clothes I’m wearing. The contrast against the ground, walls, and sky is more important than the contrast against a black shirt 99% of the time.
And yellow gets lost against off-white walls, beige carpets, and overcast skies. I agree that both orange and green are more visible in the widest variety of scenarios. Ordered some green string recently only because the brand did not have orange available. 
I meant that competition players wear dark coloured shirts for easy visibility of string, mainly for the judges. Not all, but most. Yes, orange and green are also highly visible colours but yellow is a nice colour and many people I know like yellow, including me. I find that yellow hardly gets lost when I play tho.
But of course, results may vary.
Neon orange is the most visible imo. Not a lot of orange found naturally in nature and it shows up well on light and dark backgrounds. The hard part is finding a bright enough orange. The Something orange string is bright, but i don’t much like the feel, and Kitty’s orange, if i remember correctly, isn’t truly bright like their yellow. For now i’m happy with Kitty XL which is bright and thick and nicely visible.
The Kitty orange is very bright! For 2 minutes. 
Sadly, it fades like no other.
Yeah for kitty orange 2 minutes=2 years of direct sunlight lol.
Orange is by far my favourite colour for string though I’m not sure why yellow is so popular; almost everything contrasts against blaze orange. That’s why hunting clothing is blaze orange because it’s the most distinguishable.
The brightest orange string that I’ve tried is Twisted Strings type E which is also a very unique playing string. Definitely worth picking up a 10 pack for anybody who wants to try different strings.
I like white strings because most offstring yoyos are white or black.
yep I love orange strings as well.