This sounds like an awesome idea.... and if I didn't have like 2k points of ork conversions already planned i'd be stealin' it
As for ideas, well it makes me think of;
Gurren Lagann!
But overall you could quite easily make them look like walking boats

not sure if that's what you'd want to do, but I certainly would
As for colour schemes, I say go with whatever you fancy, in my ork armies I often do whatever scheme comes to mind, it generally helps represent the ragtag-ness of an ork waaagh and the flocking of various warbands who are all just lookin' for a good brawl.