The vehicles look good, I think you've put a tad to much weathering as it hides some detail and makes the model look tabletop, but not display.
I would suggest looking and photographing different shapes of metal (Or finding some online) and then imagine a light source, the direction of this light source will either have to be the same for every model or each squad/squadron.
Now look where the shadow falls upon on the metals and make that your starting point. Slowly add a bit more white to the mixture (Tiny amounts almost non-visble) and slowly put each layer on until the extreme (As bright as you want the colour, don't go all the way to white unless it's pure un-painted metal.
Now do the same with the orks skin but with green. one thing to notice which is done by this ork below. Is that the ork shoulders are flat detail and going for what you would with the muscles doesn't work well there.
The majority of painters would stripe that area, it breaks up the flat and looks very sinew like if the contrast is correct. Or it looks like you've pinstriped a orks arm
The spotlights on the buggies need more work, they should look like your trucks. Secondly as a added part, assuming you want to be above high tabletop (Who doesn't) I would do some
OSL. Object source lighting takes a while to learn and get the brightness right.
In short you mix the colour it's reflecting on with the colour of the light source in this case blue. This is your darkest bit that the light hits, gently paint this on don't lather it. Then slowly but surely add more white to the mix until like before it looks right.
Someone posted a good article about it, perhaps you could fish it out if your intrested?
Orks look great with bits of freehand pick a symbol and make it universal across all your orks, for example checks on the boyz. The majority of boyz have a shoulder plate or a loin cloth, put some checks on then if on the shoulder pad do some weathering if required. Another good buy would be the forgeworld rust pigments. (You will need to seal them with lahmein medium, but be careful it spreads around a bit when applied, wait a day or two before sealing)
The wires on the
KFF don't look dangerous enough and are extremely flat, I would suggest throwing the cliched yellow and black danger wire.
It's just black with some coats of yellow then a darker coat of yellow highlight to bring them closer together and make it more seamless.
This all assumes brushwork.
Mastering a airbrush is what gives the majority some amazing results fast. Check nucleorsaur. (Hope I spelt his name right)
Nards nids are also a good example, the amount of layers required for what he does works. Watermelon effect isn't bad if controlled and done properly his are a good example (It's not true watermelon though

)
Anyway hope this rambling helps!
(Edit)
I forgot to talk about veins in ork arms, which the photo also demonstrates well, it's pretty much the same as
OSL. You get the green a mix in a bit of blue and paint that on the applied area, it's got to be subtle and not stand out though. The eye and sub-brain (Brain fart moment) will pick up on it and it makes it more "real"