To start off, I completely agree with you. Zelda has awesome alternate colors in this game. There honestly isn't one I don't like. They all look very sharp. It's cool how designs from past Zelda games (Ocarina, TP, SS) are integrated into her LTTP / LBW redesign.
To answer your second question, it is kinda surprising they left a Hilda color out. There are a couple reasons why I think they didn't put it in.
1. There wasn't room for it. They wanted to keep the references from past games and had to relegate the TP design to an alternate. They also had to keep the references to the original, Ocarina, TP, and SS respectively. Then there is the dark costume, which is a common theme for most characters in Smash. They wanted to distinguish the dark outfit from every other color she has, therefore it couldn't be a Hilda color. I don't think they could have used that color as a dark one since it doesn't have dark elements in it. For the Zelda characters' dark costumes, the revolving themes are gray (or white) hair with their clothing being mainly black. The Hilda costume doesn't have those elements. If they did put purple hair in the dark outfit, it would blend in too much with the black dress, and not contrast it. At least, that's what I think. I know it's not a universal opinion.
2. This also isn't as sure as the first reason, but maybe they thought a Hilda color looked too much like her TP design (dark hair, purple front-dress piece color). I also think the TP color took priority over a Hilda color because they wanted to make sure all of Zelda's appearances in the series were covered, along with the unique dark color.
There are a couple ways I think a Hilda color gets in - 1) Ultimate has DLC color changes (which I kinda doubt, but would be pretty sweet), or 2) if we get another Smash game in the future and Zelda's LTTP / LBW design stays (which I assume it does considering its popularity) and they expand the number of colors a character can have.