Well, one reason is so there can be more than one girl on the team. The other is because some people find yellow a feminine color.Same reason why some of them are black or green.
Producers decide on a colour probably based on personality s or just random.
If based on personality then it could be like this: Like recently (Gokaiger, shinkenger Go-Onger) the green guy is more of the younger, still a child , worst type of fighter guy. Blue in the same shows is the more serious dude.I dont know. Probably to not indirectly copy super sentaiBecause America thinks teams should have more than 1 girl
Because general the guys that American-ized Super Sentai want the ranger colors that are more-so related to girls to be girls. This increases sells of toys because girls have figures to buy just as boys do. That's my opinion on the matter at least.
In North America, Yellow is typically a feminine color. This has resulted in the English series generally making Yellow Rangers female, even if the Japanese counterpart was male. Plus, on average, it evens out the basic 5-person Ranger squad to 3 males & 2 females.In American culture, yellow is generally thought of as a "feminine" color, ergo, while some sentais treat yellow as gender neutral, the USA prefers to think of it as a female color
To give a more equal balance between male and female rangers (3/2 rather than 4/1)
(Asker's comment: Well, it'd look akward for them to don't have skirts, though. Helped to seperate the answers so others can see clearly.)