how come ariel is the only child to look like her mom and have her dad's eye color while her sisters look different i don't get it how can two red heads produce one red head a bunch of blondes,a black head and brunettes?

I would say that Triton was depicted as a brunette and reyna Athena was orange/redheaded. In that case as red headed appearance is a recessive trait (meaning you need two copies of the gene to express the red hair), they could have any of the mga kulay pictured as the majority of their children would not get the double copy of genes especially when you take into account it only occurs approximately 25% of the time and that each child would restart with less than a 0.25 (25%) chance of getting red hair. I included a litrato for you to demonstrate this.

Well my cousin"s mom"s hair is black and her dad's hair is dark brown and her hair's bleech yellow.

Due to king Triton having white hair and her mother having red hair it opens up genetic options as when combined they can end up having, red heads browns and blondes which each sister has, due to ariel being the main focus they probably gave her the most distinctive feature from each parent. It is a common thing to have multiple coloured hair as it all comes down to genetic and nakaraan family members and skin type

Hope this helps :)