The blue part is the balhead or "head" of the downtube.
The red and green parts are needed to guide the forces to the inner ring of the ballbearing when the frontfork is installed. Without them you would tighten the balls in the ballbearings.
Thus the inner ring of the ballbearing is jammed between the red and green part. Note: the ballbearings are not shown.

Click on the text for the photo of the Peter Lynn Competition with: extra wide "midi" wheel and front fork , frame tube to downtube connection , tube-in-Tube connection.