You don’t need very much speed to get around a 180. I’ve been working on getting my slides faster, but for first learning them, you want to find the balance between too slow (hard to initiate the slide) and too fast (dangerous) and go from there. If I had to estimate, maybe 10mph, and focus on putting a bit extra weight on your front foot, hooking the rear foot on the rail that you want to push around, giving a good steady push out on that back while swinging your whole torso around in a fluid motion. It takes time but some harder duro, round-lipped wheels will make it easier to learn since they grip less. Once you’re comfy with the motion, you can go faster, harder, longer, bad-asser… it’s something that can always be progressed farther, with no real upper limit. Good luck!
I had the same problem when I was learning toeside checks, and this still happens - really frustrating. For me, there were a few main things that helped. One of them was to make sure to ALWAYS keep looking downhill. I find that my body will follow my head, and the times that I mess up toeside checks and go around 180 are usually when I put my head down and stop keeping it pointed downhill. If your head and chest are pointed downhill, this will naturally keep your body corked and counterbalanced, such that it wants to twist back to facing forward. If you stop pushing out your back foot/deweight it while pulling your front foot back towards center, this should help snap the board back. I hope this helps!
Hmmm… I’m not positive with a D90, I’ve never really used Nikons before. However if my white balance is coming out weird, it’s usually a result of a pre-set color balance used in the wrong conditions (e.g. set to “sunny” when it’s overcast, leading to yellowy tones). This likely isn’t the case for you, but on my S95 there’s also is a manual feature option to set the white balance, where you point the viewfinder at a target and set the whites to neutral based on that selection. However if neither of those work, I’ll just edit in iPhoto, where the Temperature and Tint sliders allow for some post-color correction. If yours are too green, sliding the tint a bit away from the green end of the spectrum and more towards the red end may help. This color correction is available in a bunch of photo editing software programs. I hope this helps!