OK, so I did have some trouble using enamels on the walker. Specifically, I noticed the upper hip hinges had cracked as you can see in the picture. They didn't crack in the front, only in the back, but it loosened the tension in the joints and I ended up gluing them. I believe any time enamels/spirits pool in the joints, it can cause the plastic to stress fracture. HOWEVER, if you are careful in using only a little enamel at a time and only a slightly dampened thinner brush (for stumping) you shouldn't have a problem. Just don't let it pool and maybe stick to something clay-based for washes (if you like applying washes thickly.
For the filter, I used Mig AMMO Blue-Grey wash. It is semi-mild, and I was careful to lightly apply it. I then added light grey paint chips via a sponge. I then came back with Abteilung oils to create all kinds of streaks, highlights, shadows, etc. You can see I hit some of the panels with the off-white oil to add modulation. I used the darker oils to add mud/dirt. There's even a Starship Filth color that was perfect for...well...starship filth. Then to add variation, I used the light tan to add some color in certain areas. Specifically, look at the row panels on the inner hip joints and how they are different shades.
More streaking on the cockpit. I painted the figures (even did the black piping on the helmets). And for the markings, I used part of the "M" on a USMC stencil, which I modified somewhat. I don't build modern armor, so I didn't mind cutting the M a little. In my mind, this is an Imperial way of counting. Each marking is a composed of 5 lines, except for the last one, which is only 3 lines. So you could say this unit has 18 kills or has gone on 18 missions...or maybe squished 18 Ewoks. Finally, I added a small decal that looks like numbers underneath.
I will get some more photos up, but for now, here's a finished shot: