A good effort thus far, but there's some stuff that annoys me:

The regions in red have too much contrast between the 'edges' of the plates and the middle of the plates themselves. If the edges were brought close in colour to the plate, or the plate brought closer to the edge colour, it wouldn't look as glaring. I feel this is a problem at the core of the ship - the edges of the armour plating is too harsh, making the ship look like it's been cut and pasted. The regions in orange have a weird geometry or shading that gives the ship a weird geometry in these regions. You might have to repaint these regions to make more geometric sense.
The large turret in green needs to have a better base which either greebles into the rest of the ship, or is a discrete armour plate. Right now it feels like it's been plopped onto the ship rather haphazardly, and it's hard to figure out how high it is on the ship. Specifically - is it underneath the armour around it? Is it above the armour around it? Are the greebles on the same level as it, above it, or below it? Right now it needs to be 'fit' into the ship.
The centre of the colossus in Blue needs some work on the geometry of that fortress shield generator. Right now it feels both curved out of the centre, while also sitting inside it, giving it a really weird look. Adding some more shading in the gaps of the generator, while also reducing the shading at the edges (See the example to the right - I've made a bit of a hash of it, because now it seems like the middle is floating above the armour, but the shading around the generator is what I mean).
Otherwise the Not-Mega-Manta is an interesting concept, especially when combined with the twin flight decks. A combat midline carrier does seem to be an interesting entry into the ship catalogue. The Knight Colossus is definitely a nice and interesting design, although I agree with Axle - a Damper field would probably fit the ship better than a fortress shield.