The answer is

>! yellow.


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](https://puzzling.stackexchange.com/users/19547/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](https://puzzling.stackexchange.com/users/15727/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](https://puzzling.stackexchange.com/users/19547/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).