Is there a grayscale JPG format?

Question

When converting a color JPG image to grayscale, and saving it back to JPG, one can usually see at least 20% file size reduction, which seems natural.

Question:

Answer 1

A color JPEG has 3 components: Y Cb Cr

A grayscale JPEG has 1 component: Y

You can convert a color JPEG into a grayscale simply by updating the header to indicate 1 component and deleting the Cb and Cr scans.