I'm not so sure Sally Field is a rumor. If you go to Kari Wahlgren's website (under July 06's news), it says:
Wahlgren is cast as Alana, the older sister to Ariel, in Disney's upcoming animated feature "The Little Mermaid III." The direct-to-DVD release starring the voice talents of Sally Field and Jodi Bensen is set to be released in 2007/ 2008.
http://www.kariwahlgren.net/news.html
It's just an assumption that Field's playing Ariel's mother because 1.) If there were a new villainess, they'd most likely re-use Pat Carroll (as they did for Morgana), 2.) The movie's said to be a prequel of sorts which, considering one of Ariel's sisters have been cast [a character who has been given no limelight whatsoever in the first two movies] and Melody isn't returning, is mostly confirmed and 3.) Sally Field just has a motherly voice and most likely wouldn't be called on to play a small part. But who knows? Maybe there's some other older non-evil female who'll come to play in
TLMIII. Right now though, I'm betting on a mother.
As for the random hair colors, the mother herself was most likely a light-haired brunette. That way the blonde, brunette, and black-haired sisters could be linked somewhat back to her. And Ariel obviously got her's from Triton (which I think shows how much alike they really are). Of course, sometimes the grandparents and distant relatives have a part in this as well. Try having seven children some time, I'm sure you'll notice they'll have some varying hair colors, too.