First of all I don't mind if somebody uses something like this in a fanfiction.It's his/her story.
But staying with the films,my first problem with this is that while It's answers one question(the horns) It raises even more questions.If they are not even the same species then why they are there?Where they came from?Why didn't we ever see a ceratopsian other then triceratops with three horns?We saw centrosaurus before,but not a single ceratopsian which resemble to a triceratops.Sure you can make up answears to these too,but we are getting further and further from what the movie gives us,and it becomes more like a fanfiction.
To me it's like saying that Han Solo was Darth Vader's real son,not Luke.They tried to hide Vader's son from Palpatine so hard that somewhere they made a mistake,and protected the wrong child.Han's sensibility to the force just got buried within him becose of the wild life he had.Without taking the numberous books/comics that were crated along the story into account you can't say that no it's 100% wrong,but it's obvious that the directors and writers of the story were not even thinking about something like this.
Also I just can't even imagine how the movie makers would think about something like this.For example:
man1:Hey,i got an idea!We could include some younger relatives of.......Cera!
man2:Ok,we could use them for the story.
man1:Let's make them a different species!
man2:Why?Well,ok,whatever.
man1:I think it's the best that we give them horns,but make not even a single hint about the specie during a film,and we shouldn't even show another ceratopsian that's look like a triceratops.
man2:??!!??
It's a kids movie.I doubt that a single kid would think about something like this from only two small horns.If the targeted audiance is not likely to realize it,then why the heck they would include it?I just see no point in it at all.
to answer the question I think DarkHououmon is right about it.In LBT universe male threehorns grow their other two horns earlier than females.The question is answered,and we didn't created more while answering it.As i said before i see no problem with the colors.Tricia was pink(and female without the other 2 horns),not orange.Their color also look prety close to Mr.threehorn's colors.
Well, the Land Before Time Wikia says that one of them is female, while the other is male
We are talking about the same wikia where you was able to read about LBT 14 like three times.The information can came from anyone,and i wouldn't call it a trusted source of information.