The answer is
Solid round the number 1,
The #1 billiard ball is a solid yellow, and "solid" is a term from billiards.
A motley few, this road, walked on,
An assortment of characters travelled the Yellow Brick Road in the Wizard of Oz (Thanks Tom)
Yes I'm a coward now be gone,
"Yellow-belly" is a synonym of "coward".
Made of two but I am one!
Yellow light is composed of red and green (Thanks trentcl)
Submersing ship with unhinged crew,
A submarine is a "submersing ship" and the paradigm submarine is yellow (think the Beatles' album Yellow Submarine).
Twixt 'Of' and 'Gave' another clue,
"Richard Of York Gave Battle In Vain" is a mnemonic used to remember to names of colors in the visible spectrum: red, orange, yellow, green, blue, indigo, violet. "York" (yellow) comes between "Of" and "Gave" (Thanks Tom)
Never in a shade of blue,
A literal reading: yellow is never blue.
I warn of danger, please don't sue!
Yellow often symbolizes danger (think traffic lights or yield signs).