WHY

You are asking why Purple is the default color, correct?
EDIT: I feel like this is vague, only reason why I ask.

Maybe the purple color that we start with when we go into a project was foreshadowing the future of Scratch. (I'm kidding)

If you're talking about purple now being the main color on Scratch, then it was made purple so that color-blind people could see the Scratch page better!

Scratch changed the color of the website to be purple because studies show that it is the best color for people with eye impairments. In changing the color to purple, they gave more people access to Scratch, and made it more open to everyone. In addition, the color used in Scratch now has been used in the Scratch Wiki for years, and nobody had any problems with it. You can read more about why the color was change here.

Announcements > color contrast in scratch topic