Return of the K-Riddler

If you spell my color

And switch the first letter

With the next one alphabetically down

This substitution

Makes a transformation

As my color then becomes my sound

What am I?

HINT1 When I say "the next one alphabetically down" by "down" I mean "closer to A as opposed to closer to Z."

HINT2 The color is not universally accurate, but the classic depiction of this thing almost always show it as being this color.

HINT3 Yes, I am proposing that "color" sort of rhymes with "letter." Hey, it's a riddle, not a sonnet!

Comment or e-mail if you need to know the answer.

--Dan Kilian

K-Riddle

July 4th, 1777