Board Thread:Speculation House/@comment-24763871-20140825141623/@comment-4010415-20140825193513

Monty hadn't come up with Ruby's personality yet when he made the Red Trailer. It was more so a resume for her weapon, Crescent Rose. As for the slightly different outfit, it was made way back when RWBY was such a young concept. Her clothes in the Red Trailer were an early design.

And yeah, the stuff that Darkcloud said.

As for more canon-sounding reasons... She wasn't feeling like her usual self, having just visited her mother's grave.

Or... the Red Trailer was a recurring dream that Ruby has. After all...

Red Like Roses: "Red like roses fills my dreams' and brings me to the place you rest'.''" Place you rest, being Summer Rose's grave.

Red Like Roses Part 2: "Every nightmare just discloses, it's your blood that's red like roses."

So, basically, Summer's blood fills Ruby's dreams and brings her to Summer's grave. Also, "It's your blood that's red like roses" -> The beowolves' blood sprayed out as rose petals.