Are they all color-blind?

...or why one shouldn't confuse the anime symbols with reality. It's the same as seriously believing that girl's eyes have diameter of five centimeters

1. They notice only that "Saotome-kun seems smaller", but nothing about the glaringly-bright hair.

2. She had to open Ranma's shirt to be sure. Wouldn't be the hair color enough?

3. Either Souun is color-blind or Ranma's red hair isn't visible from their side of the screen

2. Happousai isn't that gullible.

5. Ranma tries to impersonate Nodoka by just tucking her hair (manga-only, Jap.vol.36) An iron-clad proof that those two have identic hair colors.

 

...but there are lots more examples of such "illogical" character behavior in the anime. On the other hand, in the manga the same scenes feel perfectly natural.